I would try an adult house for him this upcoming feeding, then try to transition over to rats. Try to get to eat two mice per feeding, then on the next feeding, do one mouse and a rat pup. It may take a few tries, but it'll work eventually. He's definitely not as bad as I was picturing, just needs some beefing up

I'd also try to keep your humidity higher, it looks like he may be going into a bad shed. If you're having issues maintaining humidity, you can put some damp moss or coco fiber under a hide and they'll usually chill in there come shedding time.