There's an inherent tension between keeping things private and having other people watching for suicidal behavior; I can't tell you how to navigate it, because I don't know any of the people involved.
You might want to ask your friend how much/what he wants you to say in response to more casual questions, or those from people who know only what was on Facebook. I can see possibilities including "he's fine," "Why not call him and ask? I think it would do him good to talk to people," some level of detail, or even "That's not my business, sorry. How about those Expos?"
He might be glad of the idea of a network of a few people keeping an eye out, but not want your entire social circle to be watching him solicitiously and maybe over-interpreting ordinary unhappiness or complaints.