It appears to me that when he retracts his glove to his chest, his throwing arm comes back in as well. Both sides seem to mimic each other. When is chest comes outward, his shoulders are going backward and getting compressed inward toward his center line. That's resulting in his throwing hand getting really close to his head when it should be laid back at max ER.
Earlier in the delivery, he looks really good when his glove arm is extended toward the target.
Another big factor in a lot of pitchers I've had with a similar arm motion is that they all seem to have hand break high in the chest or shoulder area. Ideally, the hand break comes at the waist.
When I need a pitcher to keep his glove arm extended longer, I tell him to focus on leaving the glove out there and having the glove be the pivot point that the body rotates around. This also seems to get their upper bodies out over their front knee and aids in trunk flexion as well.
Does any of that make sense? Perhaps someone else can explain it better.