That is a very difficult situation. Your control and power in the situation is very very limited. You don't know his whole history, you don't know his parents, you don't know how serious the suicidal or violent thinking is. Many of us have suicidal thoughts all the time and it is a very long way from acting on them.

You might try talking to the Mother and strongly encourage her to get him the help he needs, but if he isn't in school and stays up gaming 24/7 she obviously isn't the best Mother and we don't know what her problems are. She could turn against you.

Your only other choice is to just be his friend and let him vent and understand him and not judge. People underestimate the power in that. Given his age I would record all the chats and skypes just to protect yourself. What if the mother accuses you of inappropriate behavior because she gets mad. It could happen. Be his friend and CYA I think is all you can do. Unless you knew his real name and address and called child protective services but that is a big step to take.