I am going to add a humorous story for you and for your son which may set his head at ease. My aunt used to shoot a couple of pictures each summer of the family as well as our friends.
Last year my cousin, who's 35 now, located an old image that she'd never seen before. A graphic of the two of us collectively, when I was 16 and she was 17. We're modeling with one of my best friends from our excursions to the resort, a lad named Stephen who was a year older than me.
Jodi located this photo and thought it was such a fine image, we all seemed happy, and at first she could not figure out why she had never seen it with the other family pictures. Afterward she understood ... Obviously her mother did not need to contain it with the other images because of that. But Jodi believed it was hilarious, and she could not wait to reveal it to me. This young lad, his arms around two young girls, all of them nude, and he is got an erection as well as a big old grin on his face.
However, the stage? It was totally innocent. There was nothing lustful or improper about it, and in the time we did not even see it, so he must have covered up or done something about it rather fast. Tell your son that it is no big deal unless he makes it a huge deal. It is natural.