If it was my child, boy it would be hard to see him hopeful and to know disappointment would likely come. But I think I would have an age appropriate discussion on the complex issue that is addiction/alcoholism.. and that it affects the mind, body. And it has a lot of symptoms that we see, but he really doesn't.
I would also say its not easy to just stop even though a person wants to.. So its good sign dad knows there is a problem, and knows alcohol is causing at least part of it.. Its ok to have hope, and support dads attempt to quit. But somehow I would make sure my son knew that his dad loved him even if he failed at his attempt. Its a medical/psych/behavioral issue and its complex, not tied to their relationships and the love between them. I think that would be my main focus.