He’s in denial. He doesn’t want to face the reality. He’s the goddamn avatar for gods sake. He’s the last air bender. How could this happen to him? Of all people for this to happen to. He probably starts to see it as some sort of punishment thrust upon him by the universe.
It’s his retribution for initially failing to save the world from the 100 year war. It’s the only explanation he can come up with and as a result he no longer sees a son but a demon designed to torment him. He’s a heavy weight on his back, constantly reminding him of his failure.
You have to understand that Aang comes from a group of people where everybody was born a bender. When he first met Katara and Sokka he barely paid any mind to the fact that Sokka was a non-bender. He tried to play air bending tennis with guy despite the fact that you literally needed air bending to move the ball.
The entire time Aang’s brain could not register the fact that Sokka was a non-bender. As far as Aang was concerned, non-benders didn’t exist. There’s no way for such an inferior group of people to exist. They are lesser beings to him, so you can just imagine how strong the contempt for his son would have been.
It’s sad to say, but I don’t think the contempt stops there. As I said, Aang comes from a group of people with such powerful bending genes, that all their offspring were born benders. He’s also the avatar. He’s strong, he’s spiritual, he’s married to a powerful water bender. There’s absolutely zero reason for him to spawn a non-bender.
There’s no doubt that when he eventually discovered how much of a disappointment his son is, he began to question himself. Was he ever really that powerful to begin with? Are his genetics really all that great? There’s no way his son is so infinitesimally weaker than he is.