Amy,
As my doctor put it, continuing the at-risk behavior, especially through treatment, could hinder the eradication of the disease. The problem, though, is we never know what really caused the cancer. Drinking beer to avoid getting the feeding tube has more implications but most likely is indicating your father's fear of all this crap coming at him. This would make him pretty human like the rest of us.
As much as we want to convince someone to change their habits or behavior, we will never be able to force them to do what we want. For me personally, drinking 2-3 beers a day indicates to me that I need to do something different. It would be like smoking a joint a day "for relaxation". I learned long ago that I can't do this. This does not mean it is true for everyone.
Get your father engaged in discussing what he is going through and how he is really feeling and dealing with it all. It would not be inappropriate to find a trained professional to spend time with him about his disease. If he reaches for the beer to avoid treatment or passes out just before treatment, this is a pretty good sign that he is experiencing a pretty high degree of anxiety.
There are a lot of high-calorie shakes listed on various postings and should help.
Best wishes for you and your father.
Ed