My first grey had similar car problems. Sometimes she'd throw up, instead. We acclimated her with the method reported by a_daerr. We didn't use anti-anxiety medicine; just dog biscuits. It helped that she was extremely food motivated. After she was relatively ok in the drive-around-the-block stage, we progressed to the very-short-drive-to-somewhere-fun stage. Then gradually longer. Soon, she was tolerating rides well. She was never an enthusiastic car passenger, but she willingly hopped in and didn't have diarrhea anymore. We even took numerous long trips. Good luck with your boy.