Hi Jane! I absolutely agree w/ Denise. I am a CPST, and we see a ton of incorrectly installed seats or seats that are not the correct size or type for that specific child. MOST car seat harnesses only go to 40 lbs-- but there are some that go above that. Even before you schedule an appt w/ your local fire or police station (or any certified tech), you can check the Alpha Omega's owner's manual to see what weight the harness can support. I'm pretty sure it's 40 lbs. It's best to keep kids in a harness as long as possible, so you could purchase a seat that has a higher harness weight (I believe SunShine Kids and Britax both make seat that has an 80 or 100 lb limit on the harness). However, purchasing a new car seat isn't always feasible for every family, so your son could start using only the seat belt. You can find (slightly out of date) child restraint laws on
http://www.safekids.org/in-your-area/safety-laws
"Utah’s law requires children ages 7 & under who are less than 57” in height to be restrained in a child restraint device."
I know IL has a three-prong approach-- your child needs to be 80lbs or 8 years old or a certain height (I can't remember what the height is). However, the SafeKids site has IL law listed incorrectly, so maybe Utah's is too!
Sorry about that, anyway, every state is different legally, but safety-wise, all children should remain harnessed as long as possible. That's the safest way to travel. But at 40 lbs, it's your call! Forgot to add-- NEVER exceed the suggested weight limit on a harness, it has only been crash-tested to the limit that is listed, so it may not protect your son in a crash when he's over 40 lbs.
Please let me know if I can help at all :)
J.