Hi P.,
Try a reward (or just a distraction) and see what happens. We have a pile of toys, books, games in the back of our car that keeps my 2-year-old's hands busy... she wouldn't have it any other way! Though now that you mention it, if she were bored she would probably play with the carseat straps.
You could do a reward so that if he stays put he gets a special favorite toy to play with once home or arrived at your destination.
You could also just tighten up the straps. I have read that they should be pretty darn tight for safety's sake anyway... check the manual and see what it says.
Best of luck!