You can't control how other people will respond. Even without the depression she would likely take the break-up badly because no parent wants to see their child in pain. Plus she was probably relying on you to care for him once they are no longer able. Don't tell her about the break-up until after you have done it, that wouldn't be fair to your bf to find out from his mother that she knew. And seriously think about blocking all communication after the break-up so that they can't manipulate you back into a relationship. Plus it would give them false hope that you might change your mind. Good luck

You are not responsible for someone else's depression and suicidal thoughts. You cannot be the only reason a person has to live. This is NOT on you if he decides to end his life.

You don't have to tell him you met someone new. That's irrelevant to the fact that you have wanted to end this relationship for awhile but he has emotionally blackmailed you into staying by threatening to end his life if you leave. Telling him you have already moved on is an unnecessary hurt.

End it cleanly, don't let him beg or threaten you into staying in a relationship that is going nowhere. If you have a way to contact his parents, maybe let them know so they can help support him through the break-up. Then block him on everything so he can't try to bully you back into a relationship. He will get through it. If he does choose to go through with his threats, please know that this is not on you.