IMO, this is worse than things like up all hours of the night and even drugs. It’s a legitimate gripe worse than a “minor personality difference”. No one needs to put up with emotional abuse and bullying which is what this is if the roommate is making derogatory comments about your son’s personality and intelligence.
It needs to be dealt with but the first step in doing so is asking for help - the RA and the counseling office. ASAP.
It’s likely your son will encounter other emotionally/verbally abusive people in his life at some point. It’s a good life skill to learn to deal with it.