I agree with all of the above. Definitely include your DH in the therapy. It will help him understand that this is something real, not your DC being a "wimp" or anything like that. It will also help him -- and you -- continue to support your DS in the most effective ways.
Also, I don't think this is a substitute for therapy, but you might want to take a look at this book to see whether it might resonate with your DS:
http://www.amazon.com/What-When-You-Worry-Much/dp/1591473144
It was recommended awhile back on a different thread, and we found it helpful in our house.