Sack him. Nobody's irreplaceable. No good carrying someone, no matter how able, if they're too pissed to play. Only you, who know what they're capable of when sober, will know they're any use. The audience, who never hear them sober, will just think they're crap - and the rest of the band, too - few will notice that the rest of you do get it right. They'll only notice sonny boy messing up. As for "illness" etc, you're a band, not social workers or therapists. Horses for courses.