My trans son is 17, and pretty big(tall and looks kick-ass). Where he was working(not a paid job, it was a haunted house), there was a guy who called my son "it". (he has not started testosterone and his chest is hard to hide even with binding). I wanted to get involved and normally I would but my son wanted me to stay out of it. He threatened the guy but ended up quitting. How involved would you get in such a situation? I am probably over-reacting but my instinct when any type of bullying was involved is to STOP it.
Aw, man. You remind me of my mom.
Ok, so I'm 21 -- which feels like a helluva lot older than your son, but isn't. If I were in his place, I wouldn't want my ma to get involved either. At 17, that feeling of "I should be doing this myself" kind of starts to take the reigns. That, in conjunction with embarrassment and a fledgling sense of manhood are probably making it fairly hard for him to ask for help.
And I'd respect that. If things aren't violent (either physically or like... psychologically violent, yknow, like really damaging), I think it's ok to try and let him find his own way. The most important thing, in my opinion, is for you to be open with him about your support, and even your mamma bear feelings. Knowing that you love him, than you will always find him worth defending, that you want to fight for him -- that's gonna make more difference, long term, than you talking to his boss. It did for me, anyway.
That said, if ->-bleeped-<- starts to get ugly, raise hell. But for now, if your son thinks he's got this on the ropes, I think it's ok to step back. He's gonna fine. :)