But if your post title is an indication of what you're talking about (sliding plant leg hip), then I'm thinking you are saying that he doesn't stop his forward momentum at the hip joint once he reaches foot plant.
If that's what you mean, then given his size, he just may not have the strength to stop that much mass solidly.....yet.
He should start some squatting (and other lower body exercises) to build some lower body strength, plus he would need to do some drill work that focuses on that area of his delivery to try and correct the problem.
Or you could mean something totally different and I am just talking to hear myself talk....