I know it's been a while since you posted, but I just saw this and couldn't not reply.
first off, hugs. I've been in a similar situation -- having grown up with abusive parents and feeling that tug between wanting to wash your hands of them and the idea that they're still family, no matter how poor of an excuse for one. It's difficult and heartwrenching every time you have to make one of these decisions.
Before you have any contact with your dad, I'd kind of sit down with yourself (or even a disinterested counselor) and decide what you feel you can give him and what boundaries you need to protect yourself. That may mean referring him to elder services, as others have recommended, or helping him secure residential nursing care if they accept medicaid. Or it may mean sending him a packet of info on those options and letting him take care of it himself, or even doing nothing at all.
Just keep in mind, there's no way out of this situation without hurting in some way. Either you help your dad and open yourself up to his abuse and disregard (as well as financial risk), or you don't and you feel guilty. But, unlike when you were a child, you at least have some control over what kind of hurt you suffer. You can decide which you'd rather go through. It sounds perverse, but there's some freedom in that.
DC1 -- 2005 DD -- 2009 DS -- 2011