Oh, OP, I’m so sorry you’re having to deal with this - it’s a REALLY stressful situation for you. My DM died in December after a 5 year battle with secondary cancer and it was….a journey, to say the least. I adored DM, she was my best friend and supporter. So please believe me when I tell you that I’m not saying what’s to follow, lightly, at all.
Gently, and completely understanding that this is not what you asked or what you want to hear - it’s obvious that you love your DF. From this I assume he’s done his best to be be a good Dad. Not perfect - because no human is perfect - but loving and with the best of intentions. And if he’s generally been honest and truthful with you throughout your life, then perhaps you should believe what he’s saying now.
You said he has many medical problems. There’s no way for you to understand how it feels to be him, in his failing body. If he says he’s had enough, the only thing you can do is believe him. Since you love him and can’t make him “better”, I think your job now is to advocate for your DF and make his last part of life as comfortable as possible and make him feel heard and empowered.
It’s awful. And it hurts. But him saying he’s had enough doesn’t reflect on you - it doesn’t mean he loves you any less. It just means he’s tired and his body is wearing out.
Please support whatever it is that he wants to do and validate however he’s feeling. Often not wanting to eat is an early sign of the body/mind/spirit nearing the end and saying “Enough”. You can’t physically force him to eat. Don’t make these days a battle. Tell him that you love him and that it’s okay. You’ll need to be really, really brave. But it’s one of the last loving things we can do.