No matter how good-natured one is or how much one wants their friend to be successful, as a human being, one would feel a little jealous over their friend's success in the corner of their heart. To give an example.
When two childhood friends gather and talk about their exam results, the one who got a little lower grade than the other would feel a little jealous. Or, if a mother sees another mother whose child is more successful than hers, she would feel defeated. Her ego would be damaged.
And, as a human being, Austin was not an exception to that as well. Although he loved his son deep in his heart and always wanted his well-being, seeing his son achieving something he couldn't achieve, in the corner of his heart, he was not able to digest that fact very well, and he was a little mad over the fact that his son was better.