I wonder if it is to do with excitement, could you try and desensitise him to it? Like letting him hop in the car for a few minutes but not going anywhere, gradually building up until he's no longer busting to get in there, then start taking him on short trips around the block? Might not be practical though, if you need to take him places now. Hopefully others might have some more helpful tips for you! Can always trade you for our dog...he doesn't foam everywhere he just dribbles then spews..lol. But at least he tries to keep it on the towels we put on the seat!