He's gorgeous!!! I have rescued many Petco bettas and people always ask if I paid for him. I say "yes" and they say then you didn't "rescue" him. They just don't get it. I'm always so happy to hear about other fish that have been saved from such a pitiful existence. So good for you...AND HIM!!
I saved a king last week. He was in a small tank at first, too, but I agree with you...at least he's warm & fed. Just moved him into his very own 20 gallon yesterday and watching him swim & explore felt great.