I saw Bobby Sabelhaus's name mentioned in that his throwing motion was messed up by Steve Spurrier. Partially true in that his throwing motion was messed up but not by Spurrier, he barely ever worked with him, It was messed up to begin with and everyone tried to fix it. Plus Bobby's issues were more than just football. The reason I commented on Bobby though was he also had the low carry and big wind up your son has. This will work at lower levels of competition, but at the next level those throws become incompletions or interceptions. I have been working with QB's for over 30 years on every level from HS to Pro, but this is just my view. You will get others from various posters here.
Good luck with his progress and if I can help, let me know.
To sum up in one sentence, your son has a lot to work with, and a lot to work on.