Legally, he probably has a case. Morally, what did he expect USC to do - keep one of their highest paid employees, responsible for winning games and teaching young men, on the staff? Hard to be a good example for young men when you're battling alcoholism.
Legally he has almost no case. USC can show that he wasn't meeting the legitimate business expectations of an employer by coaching while intoxicated, or representing the university while intoxicated.
They can fire him for that (At-Will employment) and that's perfectly OK. A "disability" will come into play if he was NOT abusing alcohol/drugs while employed. You use, you lose the right to claim disability.
