NO ***EDIT*** His body is NOT ready to be FF.
Get a convertible car seat and there will be more room.
I recommend: The Fisher Price Safe Voyager Deluxe.
RF 33lbs FF 55lbs
My son was not too big or anything, but we switched him into this seat at 4 months old for mere comfort. We are doing extended RF. Right now my son is almost 12 months at 23 lbs and is doing wonderful RF. The longer, the better. Some kids are RF after 2 years old! Whatever you do, please keep your baby RF longer than what they say. It is best.
When looking for a car seat, pay attention to the weight and height. The one we got is made by Britax, but costs just a little cheaper. Britax makes their car seats with high limits to allow the child to RF longer. Up to 35lbs. Some don't go that high. They also can FF longer too. And a child in a 5-point harness is much more protected than in a booster seat with the lap/shoulder belt.
These are some things to look at when buying. You have to think, "Will this last my child until she/she is really ready for a booster?" When I asked myself this, I said yes with the seat I bought. A lot of people turn their children way too early. Even a year is early. Just because they are happier or their feet touch; these are NO excuse. Better to have squished legs than a spinal injury.
No matter what convertible car seat you do happen to buy, the safest car seat is one installed properly. I suggest that you go to the Department of Transportation to have it installed professionally. We did this and it worked well for us.
P.S. Just because your state doesn't have a law doesn't make it safe. I want to just scream!