The best thing to tell him is to just ignore/avoid them. They get satisfaction out of seeing people upset.
If that doesn't work, tell him to agree with every horrible thing they say and laugh. That will seriously throw them off and they don't want to make him happy, so they'll stop.
If that doesn't work for whatever reason, tell him to tell them to fuck off or he will get someone to fuck them up. If they ask if it's a threat, make sure he tells them it's a promise.








