New CAR SEAT LAW 2014 - from Another Mom's Post....

You can still use the latch system, you just can't rely on it and should be using the seat belt past a certain weight, some believe both together is a lot stronger then just one or the other.

Here's the thing. What is actually safe and what the laws require are vastly different. It's made even worse in that states all have their own set of laws, some meet the bare minimum and others exceed it. I'm going to give you more of an answer than you're asking but I want to make it clear.
Seat belts are designed to fit a person that is 4'9" or taller. A child's bone structure isn't as well formed as that of an adult. This is why it is considered unsafe for a child to ride in the front seat with an air bag before the age of 12. I've read it should really be closer to 15 when the bone structure is more fully formed and can take the impact of the air bag were it to deploy. This is also why adults that are shorter don't need boosters, their bone structure is fully developed so it doesn't pose as much of a danger as it does to children.
A child needs to be in either a car seat or booster until they are able to sit in a car with their backs straight against the back of the chair, bottom all the way back in the seat, legs going across the seat and bent at the knee and feet touching the floor flat footed. This is when the seat belt can safely protect them without any assistance from a booster.
Age had absolutely nothing to do with when you should move them to different types of restraints. Size is more important.
It is safer to keep a child in a 5 pt harness for as long as you can. Each car seat has it's own set of maximum height and weight limits. Boosters should be used when they no longer fit the upper limits of the car seat. You can buy car seats with some very high limits now such as Diono/Sushine Kids and Britax. Some boosters can be installed using the LATCH. The LATCH system is used for nothing more than keeping the seat in place so it does not become a projectile or slid out from under the child, it is not being used to restrain the child, the seat belt is. The weight limits of LATCH do not effect installing a booster. Using LATCH as your only method of anchoring a car seat does have a weight limit that takes into account both the weight of the car seat and your child combined. Some car seats weight more than others such as ones with steel frames, thus shortening the time you can use LATCH to anchor it and having to use the seat belt to install it instead.
I'll be honest. We never used the LATCH to install car seats even though we had it. We've always used the seat belt to install it and have only used the LATCH to anchor the boosters to keep them in place. The LATCH never looked sturdy or safe enough to us. We also never used a detachable infant seat as the idea that the base was the only thing attached to the car instead of the part the child was in did not strike us as being safe. The largest recall of infant seats was because the unit could detach from the base unwantedly. We instead used convertible car seats that could be used rear facing then later front facing.
40lbs was considered the minimum weight to move to a booster. So, your son is OK in a booster at his weight but he'd be better in a 5 pt still. If your booster can be installed using LATCH than do so as the seat belt is actually providing the restraint in the event of an accident.

http://www.safercar.gov/parents/RightSeat.htm#

ETA: I forgot one important part! When checking if they can ride with just the seatbelt, the seatbelt needs to sit across their hips, not across their stomach and come across their shoulders, not across their neck. If it doesn't fit properly they still need the booster to make it sit correctly.

If you are a safety nut, he should be harnessed as long as he meets the height and weight limits for his seat. My son is 6, 52 lbs and 48" and he still sits in a 5-point harness. His seat is not installed with LATCH.

Whether your specific booster should be used with or without LATCH depends on the seat you have. You'll need to check your actual seat recommendations or call the company.

Use the 5 point harness with LATCH for as long as you can (to the harness's weight or height limit).

Then when you switch to the seat belt, whether or not you are supposed to use LATCH depends on the seat. Read your owner's manual carefully and if you don't have it anymore, look for a copy on-line.

what is the new car seat law???

Check your car seat restrictions, but my car seat says to stop using the LATCH at the bottom of the car seat at 40 lbs and use the seatbelt to hold the car seat in place. Everyone is right: 5 pt. harness is best for as long as possible.

You should keep him harnessed to the limits of the seat to be the safest...

I would read your safety manual... Usually, you do not want to use the both the seat belt and LATCH together; but SOME combination seats (5 point harness to belt-positioning booster) allow you to use LATCH to secure the seat, and the seat belt to secure the child.

ETA:
DO NOT use the latch system to the limitations of the harness system... Use the seat belt instead of LATCH system to secure the seat to the vehicle (while using the harness to secure your child to the booster...) if your child and the seat together exceeds 65 lbs... While the seats harness is made to hold your child to the limit in the manual, the LATCH anchors in the vehicle may fail. (See the previous post about new 2014 seat belt rule...) for example... My daughter's harness will keep her secure until she weighs 55 lbs... But at that point, her weight plus the 15lbs the seat weighs would be too much for the LATCH system at 70 lbs; thus, instead of using LATCH I would use the seat belt to secure the booster while keeping my DD harnessed. Once she outgrows the harness, then I would refer to my seat's manual as to whether it is safe to use the latch system to secure the seat while using the adult seat belt to secure my child.

Also, when using the harness, DO NOT use seat belt and LATCH installation together... The seats are made to handle crash forces in a certain way; using both to install means that in an accident the seat is being restrained with pressure being applied in different spots, instead of being concentrated where one system or the other needs to be; which could cause seat failure. Also, the belts restraining the seat are made to have a certain amount of give when facing crash forces... Using both systems can actually restrain the seat TOO much.

Many 5-year-olds are not ready for boosters, and he does not weight much. Is he squirmy? But if you choose not to harness any longer, make sure it is secured properly either way.

If he is in a high-back booster, that is a true booster. Many REQUIRE a headrest (and the booster itself is not sufficient). Please check the manual or call the company.

You can latch some boosters, but it has to be allowed by the seat. Again, contact the company.
Good luck!

Most kids I know went into a booster around age 3. A booster only serves the purpose to raise the child up where the seat belt will fit correctly across their chest and not hit them on the throat.

That's why a larger child can sit in them. They are as safe as an adult. I don't think it needs to have a lot of bells and whistles. It's purpose is to simply make his body higher on the seat.

In Oklahoma kids don't have to be in any sort of car seat or booster once they turn 6 so maybe it's because of that we put them in boosters sooner.

Drive to your local Fire Dept or Police Dept and they will check it for you. Good luck.

I just make life easy and go to State Troopers for clarification and installation. Why second guess yourself.

Yes, I agree with JB.
The safest way is in the 5-point harness. So as long as he is still under the weight limit specified in the car seat manual, you can keep him that way.

I moved my son from the 5-point harness to a booster last spring, just before he was turned 7. He would still be within the weight limit, but I needed the 5-point harness seat for my younger son and at age almost-7, a booster is typically considered to be ok (not as good as a 5-point, but ok).

My son was in a 5 point harness until almost 6 years old, and it never bothered him. He knew how to buckle and unbuckle himself, so it wasn't any trouble to leave him harnessed. He's now 7 and has been in a backless booster for a while, but will have to go back to the high back booster with seat belt when the laws change. Rather than switch then, I've already put the high back booster back in the car, so he gets used to it.

My daughter is 4 and still harnessed, even though she's asked about when she'll be able to use a big girl seat belt. She'll stay harnessed for quite some time since we'll be struggling to figure out how to fit 3 car seats across the back of our car when the new one is born (can't afford a new car haha). I've also heard that they may be changing the laws to state that children need to stay in a 5 point harness until age 8, but I haven't seen anything to substantiate it. If it's true, I think it's a bit much.