So very sorry for the both of you. Hugs to you both.
Mental illness IS a very serious disease as I am quite sure you already know.
He needs to see a professional ASAP.
He severely needs to be on some medication as well as attend therapy sessions with a licensed therapist who has lots of professional experience dealing with depression.
If he refuses to take the necessary steps to get the help he so desperately needs, then it sadly will be up to you to do it.