Report the criminal conduct to the police. After what happened at Penn State with the Jerry Sandusky matter, you know all too well that those in the know that do not report can be held criminally liable (President, Athletic Director etc....)
Some people can be held liable both criminally and civilly. But regardless, you need to go to the authorities with this information.
I am licensed only in Texas. Offering information of a general nature in response to a question is not intended to be legal advice in your state.
As a parent why would you even hesitate to go to the police? But, you'd better be correct. If he is not a convicted sex offender you will bring the world down on yourself.
DISCLAIMER: This is not specific legal advice and does not establish an attorney/client relationship.