M.V.
I am sorry this happened to your son. It seems you have an opportunity to do some good though. It sounds like you are seeking the right thing to do and the right way to do it.
It is likely these children have also been preyed on by others and your tactful approach could very well help them and their families address and turn around painful, shame-inducing life patterns that have begun.
Definitely check with local law enforcement about what to do because you never know if the problem itself might be in the acting-out child's home.
Also, I believe you will get further by being in emotional control and logically explaining your concerns while remaining focused on the issue or behavior that needs to be addressed. Especially if you want to be taken seriously and not be seen as a raving threat. I have found this to be true both with talking to local authorities or to parents.
Sexual bullying is an unfortunate problem. To become someone who is eloquent and capable to stand up for what is right, the right way is an awkward transition to go through at first, but the more people there are who can stand up for healthy, uplifting, respectful behavior, the more people there will be who will recover from degrading, insensitive, and humiliating lifestyles.
You can also speak with local elementary school counselors in an anonymous way that will lead you to getting more information on the right kind of steps to take. If you find they are also perplexed or unwilling to help, you can go to the district level.