The nerve does lie on a groove and the slipping back and forth causes irritation and eventually can result in nerve damage. Repetitive motion in pitching doesn't help. There may be bone spurs there as well.
However, the problem may actually begin at the cervical or shoulder area, so make sure that a full examination is made to determine the exact cause. An MRI is done to check that the ulnar nerve is intact. And note, this is more common in patients aged 35-55 or those heavily involved in sports. Do not let any doctor tell you that surgery is necessary at age 15 (which would be ulnar transpositon), there are other steps to be taken first, which might include splinting, steroid injections or occupational therapy.
If discomfort is occuring while pitching, you might have to make sure he is pronating rather than supinating his pitches. In other words try to define why it is happening before you take a bold step.