I’ve has “some” side arm pitchers to coach (manage really) but I’ve had absolutely no experience helping these people in there careers. All of them came up the ladder pretty much by themselves and all were self taught.
The reason for telling you this is that sidearm pitchers ( slingers in my day) are extremely challenging to bring along. In my opinion the body has to really support this style, way beyond any other athletic movement(s) that I’ve ever seen. Then there’s days rest… how much, when, why… and so on.
The coaching personnel that handles these people really have to know what they’re doing. I mean really have to know what they’re doing. I say this because of the lumbar/spinal demands that this style places on a man.
There is a person on this web site that I have a great deal of respect for on this matter, and that’s Zita. I would seriously suggest keeping her posted on what your son is… and is not doing for training and playing. Where your son is going… she’s been there, and you couldn’t ask for a more honest, straight forward, hip shooting individual on the subject of sidearm pitching. In fact, try and post some video for her to look at.
So, monitor your son closely with his sidearm training. This is nothing to leave to the “want-to-be” that want to make a buck holding some kind of camp on the style of sidearm pitching.