Why are 13 year olds lifting weights rigously anyway?
An MRI should be done to eliminate a tear. All throwing, even lessons come to a hault.
Do your homework on shoulder surgery for 13 year olds, you won't find too many doctors who will do it, even the best because shoulder surgery is very difficult to fully recover from, small tears can take care of themselves over time. If it was mine, no way am I going to put my 13 year old through major surgery. No more pitching.