I think my betta instantly got swim bladder disorder.

Hi! New member here. First post.

This just happened perhaps 30 minutes ago. I left my room to do laundry and when I came back, my nephew had my betta fish tank open and had dumped a lot of pellets into the tank!
I do not really know how many my fish ate but when I got there, he had eaten perhaps 4 or 5 before I could clean them out. He has surely eaten more.
Almost instantly, he started swimming on his side and floating to the top of the tank or sitting on the bottom. Right now, he is floating upright motionless in his tank but whenever he tries to swim, he tips over and flounders.

I did a perhaps 30% water change simply to get the remaining pellets out of the tank and I'm currently leaving him there with the lights off to rest.

Is he dying? I am so scared. What can I do to help him?