Don't know why you'd feel the need to say anything. Based upon everything you said and his reaction it was not something he did purposefully and may not have even realized he could be seen from the outside. If he clearly seemed embarrassed, best to just let it go and never mention it to him unless in the unlikely event he brings it up first.
I witnessed unexpectedly nudity several times when I worked at hotels. Some I may have felt didn't care or it was even on purpose, while with others the person was clearly chagrined. No matter the impression I got, I never felt the urge to mention it to them should I have run into them again later. I would act the same way if it was a neighbor or other non-work situation as well. A does not always equal B and seeing someone nude unexpectedly does not always mean they wanted to be seen or have a sexual interest in you; most of the time it is just an accident. It also is a matter of context and he could have felt you were purposely watching if it seemed to him you were standing there staring before he noticed you. In the situations I've experienced I always made sure after I got an eyeful to instead act like I didn't and quicky move on, especially before they noticed, if they didn't right away.
So say nothing and whatever you do don't be a creeper and create situations where you "just happen" to be walking by his house or window more frequently hoping to catch another peek. If it was truly an accident, you might create more serious issues.
