The strings are pro hurricane and the pain is in the inside.
One of the strong high school boys I coached a couple years ago ran into that same elbow pain when using a standard Pure Drive strung with full beds of RPM Blast tensioned in the mid 50's (I think). Once it seemed as though that pain was steadily progressing (getting worse) and not just the result of one bad day at the courts, I cut out the poly and strung his rig with a full bed of Prince Premiere Control - probably tensioned in the mid 50's so that it wouldn't play super soft.
Before this happened, this guy had been doing some preseason workouts to get his game up to speed before the spring season. One thing he was doing with another coach was working on a harder serve. Before this point, this coach had been focusing on a spin serve that this young gun could land at a decent percentage. My suspicion was that his sessions of harder serving in the colder spring conditions - which turn poly string beds rock hard - really wailed on his arm and golfer's elbow showed up rather quickly as a result.
It literally took only a few days of playing with the full multi for my guy's elbow to start feeling better. Since he was a "farmer strong" high school junior who could really rip on the ball, we used 15L gauge Prince multi just to get some decent service life out of those strings. Even though that thicker multi only lasted about a week and a half for him, he played out that season with a healthy elbow.
This is only one of several episodes I've seen locally where a player using RPM Blast has run into arm trouble. I've seen enough to be convinced that this stuff is garbage.
Even if you don't get a different racquet immediately, get rid of the poly for now and at least try synthetic gut at a reasonable tension to see if you can get any relief for your son. Those racquets can be substantially easier on the arm with a less rigid string.
If you do go racquet shopping, I agree with the recommendations for a heavier alternative. This kid I mentioned got rid of his Pure Drives after that season and took up with the Wilson Six.One 95 (16x18). Shortly after the switch, he couldn't believe how much better off he was with a heavier, more stable racquet. He's been completely comfortable with these racquets, even after eventually switching into a poly hybrid tensioned in the high 40's, and that worked great for him through his senior year.