How would you handle this?

I read an article about these sorts of things a while back and this is pretty common among younger children of different es due to curiosity. This actually happened to a relatives child, and she never left her children unattended since. I'm sure it is a stage he will grow out of, but I do think it's "better to be safe than sorry" as you said. I'm so sorry I can't give you much advice, because I'm not sure what I would do myself. The best thing I can say is to keep reinforcing that you keep your hands to yourself, and that he and his sister both have areas that are to themselves only! Best of luck with this.