I don't know a lot about bullying, but I know I have no tolerance for it. I would get your son support and/or remove him from the situation ASAP. Nobody should have to feel unsafe emotionally, ever, and as we have seen in the news so many times, children at this age haven't developed or learnt the tools necessary to process and deal with these things appropriately, yet. and they can end up internalizing. IMHO, you need to get him support for this right now, before he turns to drugs or alcohol, or worse, to self medicate. Also, it sounds like you're doing a great job of raising him, being able to get him to open up about it.
