My twins are now 8yo so I've not had to deal with that for sometime now and I'm not even sure what carseats there are, but if you don't find another kind, you've got to make him understand this is not ok. I don't agree with the other responses saying to do this or that to the seat belts to fix it so they can't undo them. Sorry that is the easy way out and it's the totally unsafe way. Their seat belts need to be done the way they are made to be done for safety reasons. What if you "turn them around backwards" or "safety pin it" and then you're in a wreck and something happens to your child, you'd never forgive yourself. While your driving if and when he does this you pull over immediately, no matter where you are or what you're doing. You've got to pull over tell him this is not ok and you're not going to start driving again until he does it back up and leaves it alone. This is just like anything else, he's got to know that this behavior is unacceptable and you've got to be consistent and firm about it.