I've lived the last year with constant suicide threats. I took all of them seriously but did know and doctors confirmed that it was more of a ploy to get my sympathy and get me to do what he wanted. They have a hard time when they feel they are loosing control of us and that's the last ditch effort to pull us back in. unfortunately sometimes those threats do lead to action so i would never take it lightly. why he would tell you about it well after the fact really could be a control issue.
You have offered to do everything but go to rehab on his behalf so I think you've done more than enough to try to help him. Glad to see you standing firm for yourself and removing yourself from his problems.