Hi there Kim I am so sorry about this! Nothing makes for a more stressful car trip than vomiting from motion sickness. Just like it started suddenly it may stop (cross your fingers), there are a few options you can do. In addition the altitude or state of the roads (usually curviness) can influence whether he gets sick or not.
His own state (having eaten, being tired etc...), can also influence if he throw up. Try to take the curves slow enough so that people build up a bit behind you. Make sure to pack extra clothes (and shoes), wipes for quick clean up, and even try a towel over his carseat in the times you are pretty sure he will throw up (really curvy etc...).
You can try some medicine like dramamine (which is approved for 2 and older but if really needed may be used in a reduced dose for him now),or benadryl which you could use now if you wanted to. I always recommend using a test dose at home to see what happens before relying on it for a trip. Keep the car well ventilated and try to do your best to offer some distractions like songs etc....
I again have suffered this as well (plane and car) and have found that it comes in spurts. What is worse is when you aren't prepared so always packing for the just in case is the best plan. I never used any medicine as I felt that after they threw up generally they felt much better and it was just the clean up that was messy.
One other thing sometimes you can plan alternate routes to your destination that may be less bothersome to him as generally once it happens on one route it may happen again vs. a different incline/route down a hill or up one may be easier for him. Hope this helps! Have a great rest of the day!!