alh,
My son heard a similar pop in his elbow his sophomore year of HS. The initial diagnosis from the sports orthopedic surgeon was that TJ surgery would likely be needed. Fortunately, the MRI revealed similar findings as your son, small tear in his UCL...surgery not needed, but six weeks of therapy was recommended. He DH'd the first 6-8 games of the season, and after his physical therapy was completed he was released to start throwing again...to make a long story short, he rushed himself back to fast and could not pitch the entire spring and summer season of his sophomore year. He's a freshman in college now, and I still get nervous anytime he complains of elbow pain. When the training staff struggles to take care of his elbow, I've actually sent my kid to an Acupuncturist and it has helped tremendously.
As Bulldog stated, hopefully the training staff at your son's college can recommend another doctor in your area and get him that second opinion, it certainly can't hurt. Best of luck to him.