I think you need to ask yourself some questions only you can answer. How good a relationship or how good were the feelings between you before he got this bad? Do you hold bad feelings toward him from childhood, etc? How important is it to YOU to maintain some kind of relationship with your dad?
Me? I harbor no bad feelings toward my Dad from anything in my past. And it is very important to me, as part of my own personal value system, to care for him in his old age, the best I can. I am able to recognize and accept him AS A PERSON first and am able to let go of all wants and expectations of him AS MY DAD. These things allow me to roll with the punches, handle getting "the drunk guy" when I get him, and to ENJOY my Dad when he is able to BE my Dad. I don't know if I am making sense here or not; I hope so. But please know that for me, this had nothing to do with, and is nothing even remotely similar to, the boundary work I have had to do with other people in my life, co-workers, alcoholics, drug addicts, etc. It's different with my Dad even than it was with my brother. With my bro, I had to go No Contact and teach him my boundaries. With my Dad, it was more Acceptance than anything else.