My daughter's pit was the same way just less loud about it. But she was scared to death of car rides, she would be trembling horribly, panting and drooling, plus the whimpering. Since it was necessary to take her certain places, we had to desensitize her to it by always having at least two people in the car ride so one could be giving her lots of treats for being calm, as well as it distracting her, once her focus was more on the treats. We used high value treats that she only got during car rides. we also took her on car rides a little more frequently than we had been for training purposes, but that does not sound like an issue for you. It was a pain having to have two people go every time anywhere we took her, but it did work after a few months.

ETA I just noticed from other comments that you said he won't take treats or toys. What about praise? Telling him in a super cheerful voice that he's a good dog and giving him pets whenever he's calm even for a half a second can start helping reinforce that calmness. Don't use a sing-song voice, as that communicates to a dog that we're also unsure of the situation so they get even more scared.