Do you and his sister enjoy seeing your son's erection and knowing he is hard for you both? If yes... just take the complement and help him to feel comfortable with it... I agree that it would be helpful for you all to just offer to masturbate him together .. let him ejaculate for you both and he will feel more relaxed for a time... re-accomplish as needed :) you may all enjoy this and it will be a plus for everybody.
That is the perfectly natural status for a 15 yr. old male. I don't know if it would help or not, but it certainly would be worth a try, if you or his sister, or both maybe, would help him with some natural relief just prior to going nude in public. However, the bottom line is that if you want to truely be a "naturalist", then you must accept what is natural.
I wouldn't worry about it, I had the same problem when younger. Just leave him be...it's perfectly natural for a 15 year old boy. I still sometimes get erections on the beach when nude....it just happens.
The same thing happened to me at that age it will diminish after a while it is different for different men.
If he feels safe and secure with the situvationen so support him, it's a natural thing.
For me, it took a long time before I could walk on the beach and not be sure what people would think if I got an erection. If someone had told me that it was okay at a young age it would have been much beter.
Sintha, if you are with "like" naturists, don't you think they have seen/experienced this situation previously? Most all men/boys have erections regardless of the location, situation, etc. I got them all the time in high school and not just in the locker room showers.
I've seen them at nude beaches, at the health club and swim centers and outdoor camping areas. If you must, have him masturbate (in private) before going out in the group area and see if that doesn't "quiet" him down for a bit?