Dad,
It may not be his mechanics and may not be anything the pitching coach can address.
It is not uncommon at age 11 to experience growth plate issues, especially early in a season. There are plenty of references to it on this site. If the symptoms persist and especially with any resumption of throwing, the prudent course of action is to get to an MD and have an Xray of that elbow. Throwing into a growth plate issue only causes greater separation and pain while prolonging the healing and rest period.
Good luck to your son. With the name you have chosen, if your son is a "Flamethrower" at age 11, a growth plate situation might not be surprising. Ours had the growth plate issue at age 12. He was not very big but he threw pretty darn hard. If this board had existed when our son was 12, I would have managed the issue far better than I did.