Don't get me wrong but why are you posting that on a manga side? You don't seriously expect to get any psycologically valid advice here, do you? It's kinda dangerous, too, because whatever people will tell you here might be the exact wrong thing to do towards your friend. You better try to find medical or psycological sources for your problem.
I do agree with Shirane that asking for serious advice here is risky, however the stars are in alignment it seems. As has been said before, the best thing you can do is advise him to seek professional help. If he doesn’t want to, you should tell someone who can help. There is a national suicide prevention hotline in the US (tel:1-800-273-8255) If he is in university, many have a similar line to call. If he is relatively young, tell a trusted adult (parent, relative, teacher) It is not betrayal to get him the help he needs.
And remember - most importantly- you are not responsible for his life. The most you can do is be a positive support and encourage him to seek professional treatment. At the end of the day, his decisions are his alone.
I have a friend who i suspect that has depression, time to time he text me about how bad he is feeling or how he doesn't feel valuable or useful. Also he has sent me messages of his suicidal thougths. I always respond with positive reply, about how much we need him or how much we care about him. But I always fear that some day he will eventually kill himself and that just scare me so much... As one of his bestfriends i don't know how to help or make him feel better.