There is no evidence at all that MCNs cause any leg/hip or walking issues - they can often help them. Maz had some great experiences with the MCNs actually helping Tank's hips...
So I wouldn't stress about that.

What MCNs are you using? They could be too bulky for him, so some trimmer or narrower crotch MCNs like Itti Bittis or BBH Petites could help with his comfort.