Your dad has been fighting all along. Look at what he has done. He got your house in shape enough to move. He's gone through every therapy there is to fight this and it has not been easy. He's immersed himself into the Lord in every way he could conceive to do that. How long has he been fighting this? Can you see the fight in him? I admire how much he has fought! The same thing I admired about Mom. He's facing this headlong.
Is he done fighting yet?
One of two things will happen here. Either he will keep fighting until his dying breath. Or he gives up. You're going to have to ask the ultimate question -- when is it time to give up?
Who gets to decide that? And it's a real question specifically because sometimes at its worse, it's not the cancer causing the problems, it's other stuff. An infection, the bowels stop working, (or work too much), jaundice, which might be fixable. The list keeps hitting in ways no one can imagine. When is hope for recovery ended? Because somewhere along that line, I waited for Mom to give up, knowing I'd have to decide if I'd let her. And only looking back do I know when she gave up, and realized I never let her. I would have, had I known that's what she was doing.
Might want to ask your dad what he is doing. He didn't want to take the laxative. Why was that? Because he trusted his body that it was working it out, or because he gave up? If no one asks, he won't tell.
Long ago, you were freaking that you wouldn't have your dad in five years, and when you said that five years was the best shot he had. You're there to help him through this, knowing this is likely to kill him. Ask him what he wants, and ask him if that is a long-term decision or today's decision. If it's long-term, then he isn't fighting anymore. If it's today's decision, help him fight, just for today.