Ask him to check his blood sugar. Tell him he seems to be slowing down. He will appreciate it.

I would recommend that you do so jointly with other co-workers that are in agreement. You can coach this in terms of both concern for fellow employee - and also as a performance, and thus productivity, issue. And you should immediately start (after squaring with co-workers) 1) documenting behavior and instances (preferably through recordings), and 2) research what chain of command and HR options are there within the organization that you can take your concerns to when you are prepared.

I would be quite leery about confronting an issue such as this (except perhaps in the most casual and offhand manner. "You feeling OK today, Jim?".) on a personal level. IMO - the chances for an 'intervention', or something similar, going well at your level ... Seem pretty slim. This person's superiors probably need to be in the loop.

I don't feel that I have a close enough relationship to say anything. But if its diabetic, he's in trouble. If its alcohol, someone higher up is bound to notice.

That said, workplaces are like families: dysfunctional.

his diabetes ended up losing the portion of his two legs, is now wheelchair-bound, a terrible thing to handle, although he does handle it, he does into spells where he wants to 'end it all'. But most of the time, he's got energy, keeps an eye on his vitals, and there are lots of people checking in on him, helping him out. Maybe one of you can pretend to have a diabetes scare, perhaps pass around information to others (on diabetes) in your group w/o being too obvious or such. It's too bad that you have to play these games, if anything, people should be happy to have someone voice concern about someone else's health, etc.

Good luck. By the way, my friend w/ the two lost parts of his legs, he still runs a business so it's not the end of the world.