Not sure if this has been asked/ answered already, but do they do this at all hours of the day / night all week or is it mostly later on in the evenings and at weekends / parties?
If you already speak to your neighbours generally, maybe it's worth having a light hearted chat first, explain that you actually don't have an issue with what they do in their own home (including garden) but that your son did see them in their natural state from his bedroom window, and ask them if they might consider something like a gazebo or perhaps keeping it to times / let you know about any parties so you can just manage and keep your son busy and away from the window.
Kids being kids will find it hilarious at your sons age, and to be honest, I think around his age they start learning about the human body in schools (although admittedly I might prefer that for my kids learn the human anatomy in a more controlled environment rather than an unannounced game of peek-a-bum when looking out of their windows) 🤣 but I am wondering what would happen if your son innocently mentioned it to someone because he found it funny, but that individual ended up taking serious issue with it, and they report them to the police. Would the police then jump on it as some kind of investigations into public decency / indecent exposure given that your son is quite young.
More than anything, a quick chat might resolve this, as the last thing I think either the neighbours would want is something like that going off - and also, if others getting their knickers in a twist do report it, that might mean the police knocking on your door asking to speak to your son to verify the information, which is probably not an ideal situation for yourselves to be in either.
I personally believe people have the right to live how they like, as long as it doesn't hurt or impact others. Ultimately, this is one of those situations where neither of you are being unreasonable, but you will have different perspectives and boundaries, so perhaps compromise is the best sought remedy here.
But that's just a thought & possibly something that can be sorted out with a bit of a potentially awkward (but hopefully friendly and respectful) short chat, before it all gets blown out of proportion!
Good luck OP! Please give an update, if there is one!