what about painting some of his smaller wood objects? Can he use hobby paints?
You might want to start out by challenging his left side first. A simple way is to have him toss a bean bag back and forth between both hands. Keep it simple at first. As he improves, increase the difficulty level by you tossing the bean bag to him and make him catch it with his left hand. Remember the movie "The Karate Kid"? Wax on, wax off. Make him use his left hand/arm the same way like sanding a piece of wood and then paint it using ONLY his left hand. It takes a lot of time but it's worth it in the end. Good luck and NEVER GIVE UP!