I agree with the others, I’d wait on results. Honestly, not to scare you, but sunken eyes are very serious. That is usually what comes right before death in many cases. I absolutely agree that whatever it is came from petco beforehand, possibly pet store feeders too. Nothing you’re doing would cause an animal to decline that fast. A baby should be eating a ton, if he was an older juvenile or adult it could be normal, but healthy babies like to eat!
I’m sorry this is happening I’d get to a good vet soon. Do you know how long the results take for the mail in? I’m also in PA, but I’m over in Pittsburgh. We have a decent vet here with some Cham experience, but I’m sure there’s something closer over near Philly?