Where exactly did he feel the pain? Inside or outside? Was it sharp or dull?
Try this when you start throwing again.
Take a baseball and draw a green stripe around it with a Sharpie such that when he is holding the ball with a four-seam grip, the stripe runs between his index and middle fingers under his palm and back around the ball. In other words, draw the line such that it splits the ball in half.
Now play catch with him and watch the line as the ball comes toward you (and as you throw it back toward him). This takes some getting used to but is pretty cool once you learn to do it.
Also, and as an aside, this will teach you and him to really watch the ball and will make you both better hitters.
Anyway, in the ideal case what you will see is a constant vertical green line coming toward you. That means he released the ball with a vertical forearm and with pure backspin. If the line is solid but tilted, then you know that he is throwing with pure backspin but with a non-vertical arm slot. This can also be used to check a 12-6 curveball. It will look the same but the ball will be spinning with topspin and not backspin.
If you don't see any line, then you know that the ball is twisting and/or tumbling in flight. This indicates that he either twisted his wrist when releasing the ball or did not release it with uniform pressure.