The best method I can think of is the one I hope I'll use if it ever happens to me: front him and say, I hate to mention it, but you smell of alcohol and I'm concerned.
If you're sure you're safe, then we'll call for a breathalyzer to reassure me: if you're in some doubt, then call in sick and I'll back you up and say it was something you ate.
Yes, it risks alienating him, but it provides him with a way out, and with a hint that he has a problem. If that fails, call your ops, and if they won't do anything, call the plods. It's your licence too, not to mention the fare-paying passengers whose lives depend on your judgement.