K.G.
oops! well, you can't undo what he has already seen, but it's not the end of the world!! First of all, do not shame him for self-exploration....this is normal. Just tell him that he needs to do it in his room, in private, and don't interfere. Actually his dad should probably have that conversation with him.
Secondly, since he has already seen some images and doesn't understand them, I would suggest you take this time to teach him the basics of the birds and the bees. We have a book called "the wonderful way babies are made" which describes the process from a Christian perspective, but there are many other books out there....you go look at them first and see what you think is on his level. Explain it to him from a biological point of view....this is what these body parts are for, and this is why we don't do these things with other people until we are grown up and ready to make a baby. Keep it simple and matter of fact, but answer his questions. nothing is worse than misinformation! And you can tell him not to share this info with his classmates, because their mommies and daddies need to explain it to them. At his age, he should accept that. Also tell him it's usually not something we discuss in public.
With our kids, my husband and I told them together, as a family discussion. I didn't like ending their innocence either, and that was 5th grade! But you have to deal with this problem, and that's how I would handle it.