Hi, and Welcome!
Really we like to see kids at least 4 AND 40 lbs. before boostering. So, since you're looking for different options, I'd suggest getting a combination seat and harnessing him until he gets to that 40 lb. mark (there are of course exceptions) and he is mature enough to sit in position for the duration of the ride. For some kids (very few) this happens at age 4, but for most its between 5-7 yrs. old.
So, we'll need some more info to really suggest things more specifically.
1. What is his torso height? (seated against a wall measuring from butt to shoulder)
2. How much are you wanting/willing to spend?
3. Do you have any younger kids to pass his seat down to when he outgrows it?
Some good combination seats out there are: Britax Frontier (expensive, but lasts a long time), Recaro ProSport, Graco Nautilus/Argos, SecureKid 300/400, and Evenflo Maestro. I'm sure others will be able to recommend more, but these are a great start and are known to be good for many people.
I really think this will be your best bet instead of boostering now.