I'm guessing the "stabbing" is him extending his throwing arm down and behind him (towards SS) right after he separates his hands. He appears to be hyperflexible in the shoulder and probably needs to extend back that much to create a load through the shoulder so I wouldn't try to change that. He also appears to throw like he's throwing a grenade - relatively straight arm and little external rotation (as CoachPaul stated). It also looks like he gets his glove outside his torso and down by his knee. In other words, he flies open with his glove.
My thinking is this...
I think the glove flying open is causing two issues. First, it causes a posture shift resulting in the tilt you mentioned. Second, it causes early shoulder rotation which leaves the throwing arm in a sort of "lagging behind" position which reduces the amount of time it has to do its thing meaning the throwing arm doesn't have the time it needs to bend and then lay back into external rotation. By the way, making him slow down his delivery will, IMHO, only exacerbate the early shoulder rotation issue in that he will get too over-rotated by ball release). In fact, I'd want him to speed up and get his center of mass moving forward sooner because right now he doesn't move forward until after peak of knee lift.
My suggestion is to improve his "equal & opposite" as I think this will helps him stay closed longer and avoid the tilt. To do this, change his glove side - not his throwing arm. Since he extends his throwing arm a little bit down and quite a bit behind him, have him extend his glove arm a little bit up and well to the 1B side of the LH batter's box. Then, as he rotates, make sure he swivels and stabilizes his glove in front of his torso - not outside his torso and down by his knee. Finally, this new equal & opposite position may happen in less time than his current "flying open" position and that would leave extra time in his delivery - time for something to go wrong. So we really want to eliminate that extra time so have him get moving forward sooner to get into foot plant sooner. Try having him move forward at or slightly before peak of knee lift.