He needs to interject dominance either through threats or returning the mental bullying. If people who are bullied don't stand up for themselves it will plague them throughout their lives. What is happening to him now will seem like nothing when the bullying transfers into his lovelife or work relationships later on etc. Have to stand up for yourself. I know people who have been bullied and let it continue and have permanent mental problems as adults because of it.
P.S. Nowadays everyone is so concerned with being politically correct and not upsetting anyone ever. There is absolutley nothing wrong with having 1 good fist fight to stop people from screwing with you. Absolutley zero wrong with it. There are people who do not understand reason. This becomes much more apparent as you become older say above mid twenties and onward. You don't even need to win. Simply standing up for yourself will stop people from targeting you.
