Not really sadly, unless your willing to travel and solve the problem yourself, there really isn't much you can do. He either needs to tell someone like the principal, teachers, parents etc, or he needs to stand up for himself and put a stop to it himself. I know violence is not a good answer, but sometimes, it really is the only answer in rare cases. Definitely keep being there for him to talk to and to comfort, that will always help with the depression part of it.
OR (this is kind of a crappy option) he can try to figure out what the cause is for the bullying, either the way he carries himself, or shyness whatever it be, and try to change it. Obviously if hes being picked on for looks, or something that can't be changed, but if it's just because he looks like an easy target, he can change the way he carries himself to make him seem like not so much of a target. I hate even saying "target" because bullying is so fucked up and just not right... When i was in middle school i got bullied quite frequently by some scrawny sports kid who thought he was the shit, he did it to try to look cool in front of his friends. i noticed i kind of walked with my head down, and was timid, and not really out going or anything since i was out of my comfort zone after moving to a new school.
So what i did was changed my walk, i squared my shoulders and walked with my head high, which kind of encouraged the kid to try to get physical with me so he could prove himself, but once i saw he noticed how much of a disadvantage it was for him due to his size, i turned the tables on him and he just never looked at me funny ever again, in fact, some how we became friends in the long run lmao.
So there are options, some better than others, and what worked in my situation may not work for him, but i figured id'e put it out there.