Agree on dropping in weight. I wouldn't drop below 14 though. Until you limit the roll out, you gain no advantage from any strong core, or shell ball. I would even consider a urethane or plastic ball if you think he will still get rollout at 14 lbs with a tame resin ball.
It mighty also help if he could work on his hand position at release. A softer release with more of an open hand and less turn could actually be easier on him, and may help his balls speed. Takes some work and possibly changing pitches, but if he can do it that might be beneficial in several respects.