The same thing thats used on humans works well in dogs and that is Dramamine. You can buy it over-the-counter at you local pharmacy, walmart, k-mart, etc. Give 1mg per pound of body weight about an hour or so before travel. (for example a 25 lb dog would get 25mg, a 50# dog would get 50mg, 100# - 100mg).

Another trick is to cover the kennel you transport him in with a tarp or something so he can't see out. Its the constant blur during travel that causes the motion sickness in the first place.