Generally speaking the idea of getting video up here on the website is going to be beneficial to you and your son…perhaps not so much to your sons coach though :lol:…more then likely and unfortunately he will not like the idea that your son is learning from someone else and or not doing what he wants him to do…problem is your son has to play for him so there is a fine line there; to an extent.
Things to keep in mind…don’t think of the leg lift as a swinging motion it is a load up and more so the further back the knee is brought.
Balance when done properly has nothing to do with stopping to achieve it or stopping load just so a leg lift can be at an angle towards 3B for the righty.
Generally speaking if a kid does that type of leg lift ideally you would want a higher knee/leg lift…the higher the better as long as every thing else remains in sync/control.
Whatever the case may be a pause is not good even if to achieve balance unless with real young kids who are all out of control falling towards the plate out of sync, then perhaps it is better to think balance then not. However this does not sound like the case with your son.
Sounds ot me like the coach is simply set on one particular leg lift for your son and probably all pitchers on his staff and that would give testament to those who view things differently that he is perhaps not as all knowing as he believes he is.
As others have mentioned a video would encourage some here on the forum to give insight and opinions as they relate to the entire situation.
I would bet a dime to dollar that there is a middle ground to come to that could be properly sold to the coach that would also be more beneficial to your son…for instance and guessing; say a happy medium…higher leg lift a bit more behind his back knee…or something like that…total speculation though at this point.