His infant car seat is outgrown when there is less than one inch of hard shell above his head or he goes over the weight limit. Legs hanging over the end of the seat are not a problem, so until he outgrows it in height (less than an inch of shell above his head) or goes over the weight limit, he's fine. :)
Now, that being said, you will have to invest in a new seat eventually. Here are a few I would recommend. What you want to look for is a high rear facing weight limit and a higher forward facing height and weight limit. the best seats in my opinion right now are---
Graco MyRide65- Will rear face to 40lbs, forward face to 65lbs. It runs about $150 and will hold most kids rear facing to age 3 and forward facing to age 6. Nice deep seat with amazing side impact protection, fits small babies and big kids well. I really like this seat and really recommend it.
Evenflo Triumph ADVANCE- will rear face to 35lbs and forward face to 50lbs. Its around $130 and will hold most kids rear facing to age 2/3, and forward facing to age 5. This seat has infinite adjust, meaning no harness rethread, you just turn the knob to adjust the harness height! This is a great seat as well.
Sunshine Kids Radian XTSL- will rear face to 45lbs, forward face to 80lbs, and runs around $300. Will fit most kids rear facing to age 4, and forward facing to 6/7. This is an all around amazing seat, and I wish it had been out when our kids were smaller to keep them rear facing longer. Has a steel shell, folds up for traveling, and is the highest rear facing weight limit in the US! Can also be tethered rear facing (reduces rebound in a crash).
Britax Marathon- will rear face t o35lbs, forward face to 65lbs. Runs around $170, fits most kids to age 2-3 rear facing and 6/7 forward facing. This seat is a breeze to install so if you want to move it between cars, this is a good bet. Can be tethered rear facing (reduces rebound in a crash).
First years Truefit- will rear face to 35lbs, forward face to 65lbs, runs between $150-$200. Fits most kids to age 3 rear facing, and to age 6/7 forward facing. I havent used this seat so I can't comment much on it, but everyone who has one loves it. And its rcommended b yCPST's.
You want to keep your child rear facing as long as possible. The AAP recommends children remain rear facing until they outgrow their rear facing convertible car seat or at least age 2 and 30lbs. The BARE MINIMUM is 1yr AND 20lbs, but extended rear facing is recommended by every CPST and by every car seat manufacturer. Please google extended rear facing and Joel's journey for more information.
A few more things you may not know, or is still helpful to read even if you do!
When rear facing, the hraness should come from just at or below the shoulders. THis prevents ride up in the car seat, keeping the child in the seat as best possible when rear facing. A rear facing car seat is outgrown when there is less than one inch of shell above the childs head or the child goes over the rear facing weight limit.
When forward facing, the harness should be at or above the shoulders. this prevents spinal compression, keeping the child from being crushed down in a crash. A forward facing car seat is outgrown when one of three things happens, the child goes over the weight limit, the ears become level with the seat back, of the shoulders go over the top harness slot.
The harness should lay flat on the body, no twists in the harness at all. THe harness should be snug, and the chest clip should be level with the armpits NOT down by the crotch buckle.
Both buckles must be buckled, the crotch buckle and the chest buckle. The chest buckle is a pre-crash retainer, and will likely break opne in a crash, but its there to keep the harness on the childs shoulders. But they work together, so keep both buckled at all times.
No bulky clothing in the car seat! n owinter jackets, no Bundle Me covers, no buntings, no snowsuits. These items compress in a crash, leaving the hrness too loose, subjecting baby/child to being ejected from the car seat, or being severelyi njured from being thrown forward too far. Many crashes of dead babies are found with hte car seat and the baby bunting or snowsuit still strapped into the seat, but the baby was ejected. If you doubt this, put your child in the seat with the bunting or Bundle Me or jacket, and buckle up as usual, tightening the harness so you think its safe. Then take your child out, remove the jacket, and put him/her back into the seat. All that slack in the belt is what it would look like in a crash. Use a blanket, turn the jacket backwards after buckling, these are safe things to do.
What I would do in your situation is purchase on good car seat for your main vehicle, the one he rides in most. One that is comfy and can be used a long time like one I recommended above. Then purchase a cheaper, but still very safe, seat for the other vehicles. A great back up seat is the Cosco Scenera. ITs $50 at Wal Mart and will rear face to 35lbs, forward face to 40lbs, and fits most kids rear facing to 2, and forward facing to 3/4 yrs old.
I hope this information helped and that you take the time to google extended rear facing to keep your little guy safe!