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?