I was an RA last year at my college so I just wanted to throw in my thoughts. If your son does have an RA please please please encourage him to go to that person. Situations like this are EXACTLY what RAs are trained to deal with - plus (speaking from an RA perspective) its a lot easier to deal with something like this than dealing with the situation once it escalates.
I would encourage your son to also speak to his roommate in the moment. I don't know how he would feel about using you as an excuse but I"m a huge fan of "my mom would freak out on me if she found out I was letting someone else use my laptop, and I just don't feel like dealing with that if she did find out".
Good luck!