Hi Carrie,
Now they make the carseats to where they are like a car seat but they are also a booster.
Here is one at walmart that is made for bigger kids:
Here is one that would work for you and your son.
Graco - Nautilus 3-in-1 MultiUse Car Seat, Bravo
Product Features:
3-in-1 muli-mode Car Seat for longer use
Extended 5 point harness use from 20-65 lbs
Converts into High-back belt positioning booster for use from 30 to 100 lbs
Converts into Backless booster for use from 40 to 100 lbs
3 position recline for proper installation and comfort
Overmolded armrests with easy access side storage
One hand height adjustable head support grows with the child
Open loop belt guides
Premium softgoods package with full body pillow for added comfort
Includes EPS, energy absorbing foam
Machine washable fabric seat pads
Can only be used as forward facing car seat
Recommended Use:
**20 to 65 lbs 5-point harness (this would be for now, you wouldn't move him up to the next stage until he is over the 65 pounds)
30 to 100 lbs High-back booster
$149.98
** You are going to pay a little more for this seat but in the long run it will be worth as he will be in a seat for quite awhile.
40 to 100 lbs Backless booster