I really don't think it's a mechanical problem, at least at the root.
I do think mechanically (like many of the posts) his knee is collapsing too much, sort of throwing himself at the plate.
- Have you checked with the eye doctor? Really. His vision may have changed. 2. As a parent get in full gear in the back yard and praised him a lot, real low. Don't try to fix anything or coach him. 3. Is he being pushed too hard for velocity? If so, have a 50 ball workout where he only throws changeups, and on every near strike, call out STEERIKE or GRUNT like an obnoxious ump.
My super talented 8th grader had the same issue--throwing 70 cruising just fine then all of a sudden torquing and releasing too late, collapsing knee and "throwing" himself at plate. "Fixed" it by two just goofing around sessions in the back yard. He's now a musician, won best musician in a 13 school 6A (huge schools) competition. Same exact thing...they need to goof off some.
- Miss catching a ball, act like he hit you in the nuts, then roll around, get up, and say, wow, now that was throwing through the glove. Make sure to wear a CUP!!!!!! Really, I'm not kidding. Throwing through the glove is actually really important. 5. Get up there and bat against him. Act as goofy as you can, and miss all the soft stuff. Waggle your bat, waggle your but, point to the fences, squat like Rickey Henderson, whatever.
Find the Fun. Find the connection. Remember, to him, the most important thing at his age is YOUR approval. Don't critique him after each game. Just get in the car, don't talk baseball except maybe, "wow, William smoked that one, didn't he?" And go fishing or something, and don't say a word about baseball the whole time.