>>937902387 (OP)Tl;Dr - Your son has no idea how to handle pain, and also has no idea how to strategically face his own pain.
If this goes unaddressed, he's going to feel like an absolute faggot when you're gone. He'll hate himself for not being more understanding your situation better and trying to have a positive memory to go out on. He'll know that his last message to his dad was some snarky childish bullshit, and he'll only realize it as such when he's old enough to realize that there are few relationships in life that are precious and that we think about often in our own moments of silence.
As a father, you should raise this topic with him, admit that you're hurt, but also point out that you don't want him to make a mistake that you won't be around to fix. You will have to model the bravery required to set your relationship in a way that you, and you alone, can live with by explaining to him that life has pains that are unavoidable. Death is one of them. And that his letter is a half measure that won't serve him or you.