Bobblehd-Your experience with your son and his ability to throw a majority of curveballs without experiencing any pain or discomfort could be attributed to a number of things. He may be one of those that has such a strong muscle,bone,ligament structure that it allows him to throw curves with more frequency than the "average" pitcher and never develop arm problems.
He also-from what I've read from your posts-has very good mechanics and listens to his coach(Dad) when you notice him changing those "good" mechanics. It could also be that he is not throwing the curve with as much force as he has available and therefore not taxing his body to the fullest. Or the bottom line is that you may be correct in your belief that the curveball doesn't exert as much pressure on the elbow. We all have our own experiences to make our judgements by.
Could you please explain your post on July 22nd when you stated that when your "son got tired he stopped throwing his FB."
and "if he got tired he stuck to his off speed pitches." It does seem to contradict the last statement "My son would pull himself if he didn't feel 100%." Also, in a later post you state "His coaches never let him go out if he wasn't 100%" and
"I asked him why he came out and he said he wasn't feeling 100%".
Thanks