My 11 year old plays baseball and has pain deep in his deltoid muscle when he throws. It has been a problem for about thirty days.?

See an ortho doc. It is not normal for an 11 year to have shoulder pain when he/she throws, especially pain that has been present for a month. Have your child stop throwing immediately and have them evaluated by an orthopedic surgeon. The most likely injury is a stress injury across the growth plate of the upper arm.
SHOULDER PAIN. Shoulder pain persisted for 1 month no mention if pain rx was given and if he can move his arm. He needs to see his md for exams and possibly xrays.