An open door policy is a good way to go. How close is he to his dad? Obviously a talk would be good but if not let him see you both. If you see him looking at you then that is the time to talk. Yes he is going to get an erection (not surprising with a body like yours) but that would be part of the talk about nudity and sex. Keep us posted!!
Probably best to let his Dad talk to him about your lifestyle in a very "informative" way and let the son ask questions about his concerns. You could sunbathe outdoors topless, knowing he will see you, but it all depends on your total relationship and his connection with you as his "new" Mom. Also, your hubby could walk around nude, as a quick trip to the kitchen from the bedroom, knowing son will see him, but not flaunting anything. If you subscribe to a Naturist magazine, leave one on the coffee table for a few weeks and see if it has been "moved" from the original place you left it. He may have looked through it and wondered why it's there? You or Dad could ask, "have you seen that Naturist mag we had on the table?" And, depending on his sexual maturity Dad could suggest skinny dipping if you have a pool or fairly private lake to visit. Dad's "sex" talk w/ son could include your lifestyle and explain being a Naturist is not based on sex, it's the freedom of clothing. Too bad he was not introduced when he was much younger. 6, 7 or 8 y/o's don't really care about being nude around anyone, anywhere. And, being brought up in the home of Naturists almost always feels comfortable.