Aging father with paranoia and anger issues

My father is 65 and for the past 2 years he has been aging rapidly. He is diabetic but I don't think these recent things have anything to do with it. I just don't know what to do, he doesn't think anything is wrong with him and thus won't go to the dr.

My father is very angry, and explodes a lot. Many times over little issues or perceived slights. He thinks that people are conspiring against him. To do what, no one knows. He throws a fit if he doesn't get his way and seems to think that he never gets his way or gets to do what he wants, although my mother and I usually end up giving in so that we don't have to hear his yelling and deal with his anger. His memory is pretty good. He will hold a grudge for what seems like forever. He just won't let it go and get over it. He also makes these snide comments that are really uncalled for. It makes my life (I'm 33) and my mothers (63) miserable.

My mother and I also have autoimmune diseases and fatigue from them, but my father doesn't believe that anything is wrong with us. Now his latest thing is that he doesn't like my fiancée, who I have been with for almost 6 years. My fiancée and my father have different priorities and different ways of doing things, and my fathers hates its and thinks its a personal attack not to do things his way.

We try to go over as little as possible, but at the same time, we (me, my fiancée and my 7 yo son) try to go over to spend time and see my mother. I just don't know what to do anymore. It is really getting to the point that I hate my father and don't want anything to do with him. But I can see a big argument is coming up and I am terrified about what is going to happen, so I just stay away.

Any suggestions to what I can do to get him some help?