Well, if you buy a high-weight seat, you can use it as long as he's within the weight and height limits for the seat. You don't have to move him to a booster just because he's eligible for it. It's actually safer to keep them in a harness for as long as possible. Honestly, I see that as your only option, legally and safety-wise. I'm a carseat Nazi, though.
I can recommend some great (and not all super-expensive) high-weight seats for you if you'd like more info.