
Sorry, no quick fixes. he will grow out of it in time - if you perservere. Have you tried taking him "good" places - like a walk after a drive in the car? I find with mine, they are much better once we get on a straight road with no holdups (like a motorway at 3 in the morning :D)
Seriously, stopping & starting & loads of bends seems to make it worse, so a longer drive on faster roads helps. He may never be deliriously happy in the car, but you can help him overcome car sickness by taking him plenty of places while he's young.
It's not so much motion sickness as anxiety sickness, so familiarity is something which helps.
Keeping him in a cage is good as it will help him feel more secure.
Good Luck.