My guess is that his defense is ahead of his offense at this point. This is pure speculation on my part, but, playing low A ball, they’ll let you kick the ball all over the infield as a defender if you really need the reps. By them playing him at DH frequently, I’d guess they’re not super concerned about his fielding ability.
An alternative theory is that he is having some issues with his throwing arm and they just want to rest it.
Again, I’m just speculating.