As you know, the greatest risk is that your son will be hurt, so the situation has to change. Now, that can be easier said than done, but be persistent. Reward him for complying with your expectations, by bringing a timer on the drives and every few minutes giving him some
littleLittle noses decongestant
Little tummys (don't indulge him) treat for remaining within the seat restraints. Also, if he does not comply, punish him via withdrawal of the treat but also by time out at home (or, during the trip itself). If all else fails, and to ensure his
safetyChild safety seats
Home safety
Safe driving for teens
Safety, you might have to resort to restraining his
handsHand or foot spasms
Hand tremor during rides. If you reach this point, you likely will not have to do it for very many times - he will get the point.
Then one day we both took our kids to the park and her son popped a peice of broken glass into his mouth. Within 10 seconds she had that boy upside down, cheeks pinched, fingers jammed in his mouth screaming "Drop it! Drop it!". The point being that when it comes to safety, there is no such thing as too lenient. And there is very little that is too nazi-esque.
Car safety is broken sandbox glass in the mouth. Do what you have to do to keep him in his seat. Winch him in until he cannot even shift his weight, restrain his hands, whatever it takes. Any time he remains in his seat, either by choice or by sheer inability to pull a houdini, be effusive with your praise, pride, and gummy bears. If he cries, turn up the radio.
I think I would handle this in a none valiant way I don’t believe in tough discipline I am one of the moms that never hit there not even a tap on the hand I have wanted to many times I don’t think its necessary. Ask him he want’s to get some were simple close to home like the liquor store to get a candy. Or pick up dinner at a favorite fast food place. If he says yes then tell ok get in your safety seat and I’ll get in mine. Then go, as soon as he gets out pull over. Don’t go any further until he gets back in. if he doesn’t then go straight home if you only a house or two down walk him home. Hopefully this will work you may have to do it more than once also use things that are fun for the car. Like a new audio toy hook it to the seat before you put him in the seat. That way he cant take it out of the car make the car fun with toys only for the car.