There might be some sort of muscle/ligament issue. Even if it is not technically sprained... it is probably very sore, and there might be some specific moves/motions that an ortho could recommend for him to help it feel better sooner, or to NOT do to keep any additional pain/damage from ocurring while the damage heals.
My husband stepped in a hole (looking for a lost golf ball) and fell. He did not break anything, nor sprain his foot/ankle. But he did do 'SOMETHING' to the tendons in his foot. It wasn't severe enough for them to categorize it as a "sprain", but there WAS something injured. He went to see the ortho and they fitted him with an ankle brace that he had to wear for about a week anytime he was up and about (not sitting/lying in bed, or if driving the car). Then only when he was "active" (walking the mall, riding bike, golfing, etc). He was cautioned that it could take several weeks to a few months before he was healed, and that he should probably always wear the brace for serious sports (tennis, running, etc) to prevent another injury afterwards. He doesn't of course, though. It's been over 2 years now, and he hasn't had any more problems with it.
I'm not sure where the growth plates all are in the arms, but it certainly couldn't hurt to follow up with the ortho.