You need to respect this.
He is lucid and has mental capacity to make decisions, and has clearly expressed his wish that, for whatever his reasons, he does not want to see you. I completely understand you wanting to see him to effectively say goodbye, and i sympathise so much, but his wishes as the patient, trump yours.
You know, he may actually be taking this stance out of love as a way to try to spare you.
My mum died of brain cancer when i was 20. She went from looking and acting like herself, to dead, in less than 3 months. The decline was quick and terrifying, seeing her deteriorate physically, was so painful. She went to hospital 10 days before her passing after a seizure and was kept in for blood transfusions, then moved to hospice for her final few days. I didn't get to see her between her leaving home in an ambulance from the seizure until the morning she died, and she didn't look like my mum. In that 10 days she had major cognitive decline that has destroyed my dad to have witnessed, and 16 years on i can't think of what i saw that day and the way my mum was without bursting in to tears. She couldn't speak, couldn't open her eyes or lift her arms or her head, and was writhing in pain moaning, and i have no idea if she even knew i was there.
I hate that being my last memory of her, for years it was the only way i could think of and picture her, and it is the reason i 100% support human euthanasia and assisted dying being made legal. Nobody should have to suffer and die like that, or witness their loved one suffer and die like that.
Maybe your dad wants your final memories of him to be healthy and happy, and not of how he is in his final days. Maybe he wants his final memories of you to not be seeing you, seeing him, the way he is in his final weeks/days.
I am so, so unbelievably sorry for what you are all going through, but you really do need to respect his decision, unless HE changes his mind and asks to see you. Forcing your way in will only cause everybody distress as he won't react well to it, and you'll be hurt by his reaction and never forget it.