The 14-year-old who sexually abused your son needs help and it's your responsibility to alert the appropriate authorities of his behavior. Telling his parents is a must. Don't be surprised if this boy denies that he sexually abused your son or if his parents refuse to believe that he acted in this way.
Right now, you need to show your son lots of understanding and support, including physical affection. Tell him that this sexual abuse was not his fault, reassuring him that you will not allow this 14-year-old to threaten him any further or to do him any more harm. I know that you will get your family the help that is needed and that you will do what's necessary to make sure that this 14-year-old does not sexually abuse anyone else.