I don't know much about Tourette's and this may be no use at all.. but magnesium supplements could be helpful. This is because they are a strong muscle and emotional relaxant - they have no side effects that i'm aware of, being just a mineral supplement, but they have a huge effect. They can help women who have harsh monthly muscular cramps, and they are helpful for people with RSI (repetitive injury muscle pains).
They may, if taken at night, help your son's muscle's relax from the tension of the extra muscle twitchings, and also help him sleep.