One thing that you might try, is calling him off the couch when he is sleeping for a treat. Once he gets that when you call his name in that situation, try having your son do it. Buster will start to associate your son with good stuff coming, rather than associating him with disturbing his peace.
The only problem with not letting him on the couch as a solution, is that you aren't really addressing the problem of him guarding his space. There may be a time when he sneaks up on the couch, and your son walks by again…if you haven't changed the guarding behavior, he will most likely do it again because he will value the resource (couch) even more because he hardly ever gets to lie on it.
I agree that boundaries, consistancy, and not pittying him will help all of you have a better relationship. There are lots of GREAT books out there that you could use if you just can't find a behaviorist to help. Try browsing www.dogwise.com for a start