The roommate sounds deeply insecure and unfortunately his coping mechanism seems to be putting others down. Ugh!
The silver lining may be that your son learns to deal with this sort of toxic person which will help when he meets others later on. That said, I would guess the roommate will feel the need to use this coping mechanism with others, especially if your son is no longer a handy target.
If your son cannot relax in the room I would be inclined to (have him) push for a change.