Help bees!!! My son has been potty trained more than two months now. He's 2.5. (Wears a pull up at night)

Over the past three days he has been having tons of accidents. It's not even like we can go back to the potty every half hour like at first because he'll go and then ten minutes later he pees his pants.

This morning we got so frustrated that I asked him if he needed to wear baby diapers like his little sister. He really likes his superhero undies and pull ups so I was shocked when he said yes. So here he is. Squeezed into a size three plain white disposable diaper and WHAT DO I DO?

Did you have a potty regression? Could this be some physiological problem? What should I do?!?!