Before you return him to Petco, consider looking for other local hobbyists on tropical fish groups on places like FB. I know that I have a large school of bronze and albino cories, and would adopt your cory in a heartbeat if I lived closer, so he could have friends and a large tank to swim around in. Others will feel the same!
You could also ask here if someone nearby with cories of his species could take him.
Then you know he is going to a good home.
It's not easy to give up a pet, but you know in your heart of hearts that if you cannot give him what he requires - and he really does need friends of his species, and a larger tank - then finding him a good home is the responsible thing to do.