Do you think your roommate would be open to speaking with a professional about how he's feeling? He probably won't need years of therapy to get past a painful former relationship...Even speaking with a knowledgeable expert once or twice can sometimes make a big difference in a person's outlook. Plus, as an added benefit, a professional would be able to gauge whether or not these suicide threats were sincere and need to be treated with therapy, medication, etc.

If you don't think you could convince him to take this step alone, maybe bring in one or two other people who know and care about him to support you? Parents? Other friends?

If you feel uncomfortable suggesting something like this to your roommate, or telling other people in his life about what's happening, I think at the very least you should do as Dark suggested earlier in the thread and speak with him yourself. Offer moral support, caution him that time heals these sorts of wounds, and that he won't feel this way forever. Let him know that you care about him and would be very upset if anything were to happen to him. In other words, reach out.

I have the feeling this the kind of question most people wouldn't want to answer at all. It's a totally situation answer, but if you seriously care about the guy then just let him know -you- are there and you're his friend. Do what you can to distract him from the last four years of his life. Take him out for a night with the guys or something and go do something fun. If not fun, at least distracting. Time will pass and things will get easier. Also, if he feels like he's got enough people that care enough about him he should be fine.
Still, if he tries, be ready to wrestle a knife out of his hands or something I guess...
best of luck

~Dark