I will add a funny story for you and for your son that might put his mind at ease. I used to go to a naturist resort each summer with my cousin and my aunt and uncle. This was from age 16 to 18 or so, in the early and mid 1990s. My aunt used to take a few photos each summer of the family and our friends.
Last year my cousin, who is 35 now, found an old picture that she had never seen before. A picture of the two of us together, when I was 16 and she was 17. We are posing with one of my best friends from our trips to the resort, a boy named Stephen who was a year older than me. He’s in the middle, and Jodi and I are on either side, and we’ve got our arms around each other and everyone is smiling and no one is wearing anything but sandals.
Jodi found this photo and thought it was such a nice picture, we all looked happy, and at first she couldn’t figure out why she’d never seen it with the other family photos. Then she realized … Stephen had an erection. Apparently her mom didn’t want to include it with the other pictures because of that. But Jodi thought it was hilarious, and she couldn’t wait to show it to me. This young boy, his arms around two young girls, all of them naked, and he’s got an erection and a big old smile on his face. Cracked us up.
But the point? It was completely innocent. There was nothing lustful or inappropriate about it, and at the time we didn’t even notice it, so he must have covered up or done something about it pretty quickly. Tell your son that it’s no big deal unless he makes it a big deal. It’s natural.