Personally I think you just bought a non-feeding hatchling, but you never know. He's either a problem feeder or he's been maintenance fed. I'd try either live rat pups or live hopper mice and hope for the best.
That loose skin is not normal...underfed and probably dehydrated...
p.s. don't take this as a slam on your new addition...he's a great looking Black Pastel, the seller on the other hand...