I understand conceptually the notion that IF you're truly planning a suicide that being in a relationship with someone who really loves you, how brutal that will be when you are gone. I understand conceptually that it makes sense to leave that person... but let me say that IF he does truly love you and you break up with him for some random reason now... thinking it will make him not hurt later... whenever he finds out you committed suicide, he will think back to the moment you broke up and wonder if you were hurting then and he will torture himself for not seeing it and not trying harder for you.
I think it's fair to say "don't get into a relationship" but you are already there. I honestly think the best thing for both of you... if you love him and you believe he loves you... is to be honest and tell him how depressed you are and that you are considering suicide. IF he can't handle it or leaves or whatever, then you solved your problem... but if he cares and supports and stays and wants to help... maybe the additional support of his love for you can help you see a path to improvement?
I don't know. I mean, most of my problems is a life of never being in a relationship, never being loved... so I can't even imagine truly what it would be like to have someone love me and be wondering how to handle it... plus in my case I wouldn't be suicidal if someone loved me. Even if I was still depressed for other reasons, I am only suicidal because of having no one... so for me, having someone would help me tremendously.
I mean if your options right now are silence and dying and risking causing him pain, or breaking up with him and causing him pain now and maybe more later... why not for your own sake as well as his, try being open and giving him and you a chance to see if something good could come from the discussion?