Definetly sounds like ulnar nerve.
I strongly suggest that you send him to an ortho before you do anything. Because you have let this go for a long period of time there may be damage to the nerve.
The nerve sits in a groove in the elbow. Sometimes the nerve will not stay in the groove and slips in and out of the groove. This action causes irritation to the nerve. A test should be done to determine if the nerve is still viable. The doctor may also consider if there has been any other issue that may have caused the pain.
The doctor will then make the determination on pt or surgery to move it.
My son had this surgery and quite common for pitchers but your son is young.
If you have any questions you can send a dialog.