These questions are always interesting..trick is not to assume the specifics. Ask where you guys are when you smell it. Don't assume you're on the airplane. Nothing wrong if you smell it on him at the bar on a long overnight.

If you're on the plane, and you're 100% certain it's booze, ask him if he's feeling well. Don't make a scene in front of the crew, pax etc...if he says he's fine, don't be afraid to tell him what you smell. Who cares if it hurts his feelings. If he's also acting belligerent, clearly still hammered, or puking in the trash, don't be afraid to refuse to fly until its figured out. The next step could be to call a union rep.