He had fixed the side to side by winding up looking in a mirror for a few hours lol. Then he went to a back step which worked really good for him but he started getting too fast with it and it threw off his timing. So he went back to the side step at the end of summer and apparently forgot the mirror training lol.
Not really sure what yaw mean by throw around the body. Never was a pitcher so I'm just learning. I think I understand though. His coach last year worked on that during the winter and his delivery did look, for lack of a better way of saying it, straighter line to the plate and less rotation looking. Problem was his fast ball became flat as crap. After the first two middle school scrimmages he got lit up he went back to his old way and had a good season and summer.
I think his biggest problem is, and I maybe very wrong on this, his front leg seems to act like a shock absorber instead of a brace, if that makes any sense. Which I feel slows down his upper body and why he's still so vertical at release and by slowing down his upper body causes him to throw around his body. I feel if he braced his front leg with more of a pow than a woah it would fix some of the problems and give him a little more velocity. I believe it accomplish what his coach was trying to do without killing his movement.
But that doesn't come from any pitching knowledge. I work mostly with football players and fixing the lower half corrects all kinds of upper half problems lol. Now to figure out how to explains it to a 14 year old without screwing something else up hahaha.