When I move his foot along the Y axis it slides inward as if it's moving on the X axis too. But there are no X location keys. Is there a quick fix that doesn't involve manually editing the X location data on every keyframe to make up for it?
This is because of the natural twist out on the feet.
Some rigs have the feet straight on, other rigs have the feet twisted out. In retrospect I should have insisted that the feet be pointing straight forward on this rig, but it's too late now. Oh well, but it is a common problem that you will need to learn to overcome, so it isn't a bad thing in the end.
There isn't an easy quick fix, but you can copy the x value from the first frame of contact, and then paste that x value on each of the other steps (first contact). Then repeat this for the last frame of contact. I hope that makes sense, Eric.