I would get some motion sickness medication from the vet or use one of the OTC products for humans that are safe for dogs. I would combine that with attempting to desensitize him to car travel by making short trips (after medicating him). To attempt to desensitize while he continues to get sick is likely just making the problem worse.
How old is he? IME some puppies, just like some human kids, eventually outgrow car sickness. But I wouldn't count on that happening, and I'd do everything I could to keep it from becoming sort of a learned behavior. So . . . no riding w/o medication first for a good long while.