At this age I believe grasping what each side of the plate offers during a game and during at bats is huge. Just the ability to go to either side...not pin point control, but understanding with a guy on base in certain situations I am going to live and die pitching to the inner half or the outer half. No matter the outcome, because at this age the outcome cannot be determined by ones will over another...too many mistakes and not enough depth on situational baseball are developed.
As far as mechanically, he is sound enough for his age, his body will change and grow and so will his mechanics dependent on how his body matures. But teach the game, teach how throwing inside is necessary for certain situations so that you take any fear of doing so away. No bad thing can come from learning how to control each side of the plate. But serious progress can be achieved that is laying ground work for a better and more meaningful career.