1) How old was your son when he first started pitching?
2) Has your son ever (intentionally) thrown a curveball or slider? If so, at what age did he start?
3) You said your son plays Feb-Aug but it looks like you first posted in Nov. Does he have pain in the off season?
4) When you talked to the first MD, did you get any kind of formal diagnosis? Can you get your hands on the notes of the person who read the X-Ray (or the X-Rays themselves)? Do you have a diagnostic code (see if one is listed on the EOB if your insurance company sent you one)? Did the MD say anything about "medial epicondylitis?"
5) Can your son throw a football without experiencing pain?
6) I would suggest that you go to the Q&A portion of Dr. Mike Marshall's web site...
http://www.drmikemarshall.com/Question-Answer2005.html
...and search for the keyword "medial epicondyle".
The bottom line is that this is something to take seriously. Ideally, your son has just irritated his medial epicondyle, which will heal itself over time (this is the same basic thing as "golfer's elbow"). However, in the worst case he could have an avulsion fracture of the medial epicondyle or a growth plate injury, both of which are very big deals.