He's from Petco, he's not CB. He's CH. Being CH doesn't necessarily mean he has internal parasites, but being near other snakes ups his chance of having them. It never hurts to take a fecal sample to the vet and have them run the test. Better safe than sorry.

I would move him into something smaller(6-12qt tub), 1-2 hides, get all of your temperatures and humidity correct, and leave him completely alone for a week. He may just be dehydrated, and stress can easily strip the weight off of them quickly. A stressed animal uses ALOT more calories than a relaxed animal does. If they aren't eating(because of the stress and the co-housing/improper temps/poor hides/huge cage at Petco), then obviously they will lose more weight.