Talk to the guy face to face and express your concerns - it might get you a punch in the face or it might get you an explanation.
He may turn up to work theoretically sober/clean but we all know how a hangover affects mood, judgement and ability.
If he spears in because he is unfit to fly, his livlihood will be the last thing to worry about.
If he really has a drug/alcohol problem he might just be waiting to be found out/confronted about it because he knows what he is doing is wrong.
Sneaking off to tell teacher/the boss is moral cowardice and passing the buck - ask the man yourself, if neccessary tell him others have passed comment and you are worried he might get into trouble - that might get him on your side and more likely to confide.
Good Luck