You son will learn about the differences between men and women soon enough, if he has not figured it out already. As a general statement about life on the Camino, people are on their best behavior so you do not need to worry about issues like "predators" in the albergues.
On the other hand, European adults are generally more casual about nudity than we are. For many of them nudity is "no big deal." We all have the same parts, etc. So, a flash of something private while one is changing clothing or getting into or out of a shower is to be expected.
Occasional or incidental nudity in an albergue setting is not intended to be sexual or provocative. It is just a normal part of life. People do not flaunt it as a general rule.
Trust me, most of the folks I've seen in the buff have nothing to flaunt, including me. Nonetheless, I try to be as modest as situations permit, but am not put off by how others conduct themselves. It is just a slight difference in cultures. Again, as your son grows and experiences more of life, this will just be another experience. International travel broadens the mind.
So, I recommend a parent-child discussion about healthy nudity in a gymnasium / albergue scenario. I am even going to go so far as to guess that when you raise the subject with your son, he will likely be initially embarrassed (not of the subject, but of speaking to his MOM about the issue - teens are like that). OR he, as I did with my father at that age, ask YOU what YOU would like to know...go figure.
My father just shrugged his shoulders and let me know he was always available to talk. But by age 13, I had read Grey's Anatomy, and as many books on the related subjects as I could get my hands on at the local library (in the mid-1960s). Let's not even talk about what my peers showed me privately at that age.
With the "wealth" of social media and web-based sources of imagery available to anyone with the inclination to click on a link, NOTHING surprises me anymore. But my point is that your 13-year old has probably already seen more than you would have preferred as a parent. That is not a judgement. It is a simple statement of fact in 2015.
All said and done, I think what you are planning to do is HUGE. It is a wonderful growth experience for any teenager. Your son is indeed fortunate to have a mom willing to invest the time and effort to do this. I am hoping to do the same with a 15-year old nephew of mine in the coming year. Besides, his "how I spent my summer" essays will "slam dunk" anything written by anyone else.
I hope this helps.