I agree that getting a professional involved is the best thing you can do for him right now. Stress, when you talk with him, that depression is a serious illness, and just like you wouldn't expect to recover from, say, cancer without help from a doctor, neither should you expect to just "pull yourself out' of depression without one, too. Stress that you and his other friends love him, and do not want him to hurt himself, and that includes killing himself, that you need him in your lives and that things will get better eventually.