This might be off track, but I don't think I would worry about it. If he starts drinking, and as you say everyone already knows, than people will be looking at him. Not you. I know it's uncomfortable, but I'm not sure there is anything you can do about it. If the family already knows I'm not too sure what trying to hide it is going to do.
I sometimes get embarassed by what my BF does and/or says when he's been drinking. Especially when he starts to lecture his nieces and nephews. But I've learned that I'm not the one who looks stupid when he does that.. HE IS.
Maybe if everyone sees him make a drunken fool of himself he'll the embarassement with help kick the denial to the wayside??? Probably not.. but it's a good thought.
Good Luck!!