Two very noticeable things: he’s got some serious Okajima-head. His head needs to be much more upright and he needs to be looking at the target. It’s very difficult to pitch accurately with a blindfold on. Looking at the source of the head pulling off and it’s probably his glove movement. This can happen when we try to overthrow or if we are trying to pull the glove to the chest.
He’s dropping / pulling his glove from chest to waist and his head is going with it. Glove up, head up. After he gets glove arm extension, the glove should simply flip from palm down to palm up as his elbow drops and allows his shoulders to rotate toward the plate. Get him to chase the glove to the target with his chest. I know you aren’t looking for added velocity, but unfortunately, if he fixes this problem, his velocity will increase. Sorry to disappoint.