You need to get a bigger car seat. If he's reached the limit for that seat, he needs a new one. I'm sorry to say it, but you should opt for one of the more expensive models that are 5 point-harnesses for up to 65-80 lbs. I would not put him in a booster this young.
The concerns you listed about (getting out of the seat & slumping while sleeping) are not the only concerns. The main concern is safety. Children that young are very vulnerable, their neck muscles (and other body parts) aren't as strong and could get severe whiplash if he was in an accident in a booster. He needs the support and protection of a car seat.
In my opinion safety comes first, even if it means shelling out more money. Better safe than sorry.
These aren't just guidelines to follow, but actual laws. And for a very good reason, they save lives.
I've seen the "we did this in our day, and we turned out just fine!" argument... what about the ones who didn't make it??? That's why we have better laws now, to protect kids in car accidents.