After Billy finds out that his friend Michael is gay, he's supportive of him without thinking twice.
The fact that Billy's father is willing to give up the strike to make sure that Billy can pursue his dream of dancing.
Remember that, until his father saw him dancing in the gym, Billy had been afraid his father would disapprove of his love for dancing. Jackie has the complete opposite reaction. He even calls his son a genius at it. Doubles as Tearjerker.
The final scene when his father and brother come to see him dance. Oozes So Proud of You without having to say it.
When Jackie sees Billy dance onto the stage, he's so moved he visibly gasps.