Some people are what's known as functioning alcoholics. Unlikely, but he might not yet know it's happening if someone has a serious drink problem. For what it's worth, I think society as a whole has a drink problem. We're told not to drink too much - cue happy hour, 2 for 1 offers etc etc.
Might not be noticable to the naked eye, but a drug & alcohol test, administered on a regular basis, would be infallible.
It's common practice in most professional industries.