He also just has very slow load time in his legs, particularly vertically. There was one rebound the other day where the ball bounced high off the rim, right towards him, and he looked sure to grab it but when he flexed down to jump, the ball went over his head before he could even start coming up again.
On the other hand, he moves better horizontally than I had expected. He's certainly not consistent with it, but he has shown glimpses of the ability to slide, enough to make me think that it's worth a shot at investing in trying to coach him up. The slow loading issue is biomechanical, and I don't know how much he can really do to improve, but coaching to help him position his feet better, take more efficient angles, and recognize offenses earlier can definitely help mitigate.
The way he alters opposing offenses cannot be taught. Virtually every time someone gets into the paint with the ball when he's underneath, you can basically see their "oh shit" moment, where they either kick/dribble back out or end up throwing an completely altered teardrop/scoop shot. Same thing when a big grabs an offensive rebound- his presence scares them into doing something they don’t really have the muscle memory to do effectively. If he can extend his zone of influence from 15 feet to 20 feet, without being a major liability in the PnR, he can become a defensive presence to be reckoned with.