It sounds like while your child may have trouble advocating for himself, the roommate is the one with issues.
Your son DOES need to find ways of dealing with this. Personally, I would suggest a mix of avoidance (studying in the library, for example) and firm confrontation (when leaving and asked where he is going, answering “it doesn’t concern you”). I concur with the posters above that the counseling office could help with this.
The roommate sounds like a bully and your son should discuss this with the RA, and if the situation advances beyond profoundly annoying to threatening, he should make it clear that he feels unsafe and he should request a room change. I would recommend that he keep a log of who he talks to about this as the change may be difficult to effect, and a record of working this through the proper channels may get his request more urgency.
I agree with @Publisher that it sounds like it could become dangerous. While the OP worries that a room change could make it worse - which seems impossible- the mere fact of that concern suggests that this boy may be an attractive target to bullies, in which case the counseling office is likely to be far more helpful than the RA.
Hugs to you, OP. It has to be really stressful to be watching your kid go through this.