N.D.
You are smart to realize there is an underlying reason for his behavior. Hopefully the counseling will work, but try to figure out if something unusual happened 2 months ago. Did he do something in school that would cause the kids to make fun of him? How does he react when they call him names? If he reacts at all it just feeds their bullying. I am not a fan of bullying, but over the years have observed that bullies usually pick on a kid for a reason and the more the kid reacts the more they bully. If you ask the boy why kids call him names he will probably say he doesnt know, but if he thinks about it he will be able to figure it out. Then he needs to ignore the other kids, totally. No looks, no gestures, no whining, crying or anger. Just act as if they didnt say anything. They will soon get bored, but it might take a month or so.
I can probably guess that he did or said something incredibly dumb or babyish and he is embarrassed by it. He needs to know that everyone makes incredibly dumb mistakes at some point. Maybe even relate your most embarrassing moment to him.