Agree with M0nica.
Three recommendations:
1. Encourage him to see a GP, if you can. (I realize that you have said he is not willing. I wonder if he would go if you phrased it this way: "Dad, I know that going to see your doctor isn't something you've been willing to do. However, I am worried about you because you seem to have minimal energy and I don't think you are eating properly. I'm concerned there is something causing this that the GP could help with. If you go to the GP, they may be able to do something that would help you feel better.")
2. If you're worried about the dog you need to contact the RSPCA.
3. If he's cognitively intact, the reality is that it is his right to live at risk. I recognize that this is not how you want him to live and that it is far from an ideal situation. However, it's his life.
If running his household and doing his life maintenance is too much for you, I'd recommend stepping back. You're not obligated to do for him what he won't do for himself. That's a reasonable boundary. If he wants to have a clean house, etc., there are people he can hire to do that. If he's not willing, as you say he isn't, to hire those people, it is his choice to live in a tip. That's not your problem.