Your son seems to be inconsistent in the location he plants his front foot as well as the direction his foot points. I would argue that it is likely caused by all of the unnecessary movement at the beginning of his delivery. Specifically, he has a big weight shift towards 2B as he goes into knee lift. This is caused by how far apart his feet are in his set position. Have him set up with his feet no more than arm pit apart and you’ll minimize or eliminate the weight shift.
Also, I wouldn’t get too caught up with having his front foot plant on the white centerline on the portable mound. If you look across MLB pitchers, you’ll find a significant percent who plant to the throwing arm side of the centerline. As long as he is able to maintain a stable posture as he moves down the mound, he should be ok.