I'm so sorry you're going through this. I know the pain of seeing a father dying. The truth is, you know it's coming but it's still going to hit you like a ton of bricks when death comes. The dying is harder on the survivors. Do your best. I don't know you or your son well enough, but my kids were pre-teens when my father died. I was honest with them. And as uncomfortable the words "death" and "dying" are, they are reality and my kids understood the finality of it. And I do my best to keep his memory alive.
Don't be too hard on yourself. Keep these last moments with your dad regret-free. And try to remember that everyone grieves differently. Respect that. HUGS.