J.B.
We've been giving my oldest (who is now 5) Dramamine for motion sickness since he was two. He now takes a full dose, and he's never had any issues on a half or full dose. It seems to do to him what it does to me -you fall asleep for several hours and then wake up. That's it. However, we gave it to my 3 year old, and it had NO effect on him whatsoever! I've never gotten nauseous or known anyone who has from Dramamine.
Keep in mind that he may be drowsy or sleepy whenever you stop, but that's far preferable than him being sick and/or throwing up all over your car. As a life-long sufferer of motion sickness (that's particularly awful when I'm in the backseat), I vote for the Dramamine. He's two -just put a pull up on him. We put pull-ups on our 3 year old for road trips. He still goes to the potty, but at least it's not a big mess issue if he's asleep or has an accident.