This is way above Reddit’s pay grade, but a helpful cautionary tale. Therapy for you, therapy for your child, and I agree with other posters, I would be taking space and not putting these children in each other’s orbit. It sounds like there were a lot of poor decisions made by all of the supervising adults/Obviously hindsight is 20/20 but assigning blame isn’t going to help your son process this in a way that ensures it doesn’t happen again.

Your child is not a bad or evil person but these things do happen without sex education, as you’ve acknowledged. I worry about something like this happening with my own child, in either role, though he is much younger. I am perhaps hyper vigilant about ensuring he understands those boundaries, but is also never left in a situation where he could make those impulse choices. I cannot imagine how hard this must be for you especially given that you weren’t there, for that alone I hope you are able to work through this with a therapist.