Trying to make the symptom go away with drugs or even herbs can have negative effects. I don't think that a particular herb or supplement is going to do the trick, but a more multi-dimensional protocol that's tailored to your son's needs and works more on the causative level.
If you're interested in working with a homeopathic practitioner who can do this, you can look at http://www.homeopathy.com/clinic.
They get good results treating people with DS, autism.. all chronic conditons, actually. In homeopathy it's not necessary to have specialties with respect to the allopathic disease labels, because this system of diagnosis is very different - the specific disturbances in the person's life force are treated in a very individualized way.