I'd have to know your father. Changing someone's mind generally requires knowing their mind.

Some theories that occur to me, which I would attempt to explore while talking to him about his views on life and death:

• He's sufficiently afraid of dying that seriously entertaining hope of an alternative is emotionally stressful, and so he's highly motivated to avoid such hope. People do that a lot.

• He's being contrarian.

• He isn't treating "I should live forever" as an instance of "people should live forever," but rather as some kind of singular privilege, and it's invoking a kind of humility-signaling reflex... in much the same way that some people's reflexive reaction to being complimented is to deny the truth of it.

• There's some kind of survivor's guilt going on.

If all of those turned out to be false, I'd come up with more theories to test. More importantly, I'd keep the conversation going until I actually understood his reasons.

Then I would consider his reasons, and think about whether they apply to me. I don't really endorse trying to change others' minds without being willing to change my own.

Having done all of that, if I still think he's mistaken, I'd try to express as clearly as I could my reasons for not being compelled by his argument.