If I were you, I would let him know you are always available for him when he's ready to share the issues/problems he's having. there's not much more that you can do but let him know you care and you will help him when he's ready to seek support from friends. Goodluck!
Honestly that thought cross my mind about him being biopolar, and depress, as well as other co-worker. I also thought about talking to his Manager, but I'm not sure if she will be able to discuss his situation, cause I think she has some information, cause he being taking alot days off work, more than what OUR employer would allow, so I'm thinking his using FMLA. but I will keep you guys posted.
It sounds like there may be more than one issue going on in his life.
That is kind of what I was thinking, the manager won't be able to tell you anything.
It doesn't sound like you know enough about what he might be dealing with.
I wish I had a friend that cared as much as you.