Take a closer look at where your son stops his glove. The body has a balancing system that uses the arms to balance itself, in a static state and when moving. You son is using that system, without realizing it, to position his glove (part of his balance) and thus his stride foot STOPS directly under that glove (balancing system) in order to feel confident while progressing forward and throughout his release (pitch).
To your son's credit, he can manage to overcome his balance-counter balance choreography to locate his pitches.
Ask your son to go through his delivery motion in slow motion and stop his glove deliberately where he does in the picture above and watch how - instantly, his stride foot stops its forward reach. That stride foot will plant itself directly under his glove hand. Now go through the same slow motion, but this time stretch the glove out directly forward in line with the heel of his pivot foot - or as Roger suggested, along a line to the plate. Watch how instantly his stride foot plants directly under the glove hand and in a direct straight line to the plate - thus eliminating that pitching across himself.