by the way he knows I am in a relationship, but I guess he doesn't care.
Most Helpful Guy
Cut contact completely. This guy is going to always be pestering you like this. And 'suicidal' messages are giant flags of emotional manipulation in progress. You must have zero tolerance for that or it will escalate. You are not responsible for his happiness, that is something he has to work out on his own.
This is causing you problems with your 'real' relationship, that's totally more than enough cause to cut contact with this guy. You have to be thorough, and you have to be firm. No contact in any form. Not appropriate behaviour.
Most Helpful Girl
I think that he is trying to prove to his friends that he can get any girl even a taken one so don't go for it. Just say that he is annoying you... I did the same with this guy and he stopped right away but after a few weeks he tried one more time so I slapped him.