C.J.
Your son is a little confused right now, but he'll be okay. The best advise I can give is to take a deep breath and give yourself some loving time to do something other than worry. Then next time you have a "natural" opportunity to engage with your son about what happened, look at him in a loving and interested way as though he is telling you about the picture of a tree that he painted in school today. How you respond to him is how he will feel about himself. He is looking at you for cues on how to feel and react about this. If you are afraid, feel it was shameful, or in utter confusion....he will be too. You don't have to tell him any thing. When he speaks of it, listen and look into his eyes with love...not sadness covered with a smile. When he seems to be through talking ask him questions in an objective and light manner like, "how do you feel about that?", or "do you have any questions about your body?".
Your responses do not need to be complex, but they do need to be true and real. You can say, "I hear you and I love you." or "Everyone's body is special and you are in charge of your own body. There are lots of ways to be in our body like playing, running, and cuddling with mommy.". He needs to be empowered and feel in control again, and supported and loved by you, which clearly he is. The last thing I would say is when you feel like he has expressed himself and your conversation is through, redirect him to a fun activity, give him an opportunity to be carefree and let go of the heaviness this has apparently created for him.
It's hard not to be overcome with despair when we see or feel our child has been imprinted or scarred for life by something someone else has done. It is hard to let our children go out into the world without us, it is hard to trust others with our "babies". There is no pain greater than a parents grief for their child, and their is no joy greater than parenting. Focus on your joy, let him let this pass in his own time, and trust yourself that you are doing an amazing job as a mom. You are obviously a loving and hard working mom if you are caring for three children, on your own, for the better part of the week. Bless you.