I suppose you could visit and bring it up in a general way.
The old "I have a friend who is...."
I can't think of anything, sorry.
I was much better at hiding my drinking.
He is functioning but you are his siblings and his daughter, if not him, has asked for help.
I was pissed off when I found that my family had stayed silent knowing full well that I was drinking my head off in the final weeks of my drinking.
I was stumbling around the house every night and they still chose to say nothing.
As you say, if he is hiding it, he knows it is a problem and might appreciate someone bringing up the subject. The "head in the sand approach" achieves nothing, and what if he or someone else gets hurt because of the drink? He may be functioning but he is still probably seriously impaired right?
They are just some thoughts. Sorry you are going through this.