Limping

Hi guys,
I recently built a new cage that is 5 levels made from an old china cabinet/bookcase thing with chicken wire doors. There is no where for them to fall down unless down a single ladder (each level is separate except for a hole in the floor that the ladder leads through.)
But my Wolf, my ~1yr old boy has been limping for a few days! We introduced them to it about 6 days ago and he's been limping for about 3 and we have no idea what happened.
He doesn't show any tenderness or pain when it's touched but he won't put any weight onto it. (It's a back paw)

He is eating and drinking normally and goes up and down a bit, but mostly stays on the bottom level. (One wall on the bottom is heated, but there isn't a food or water dish on the bottom level.)

I'm not sure if it's a problem in his hip or his foot? There are no visible sores or swelling or lumps or anything that gives any indications.

Thanks!