There has been some good advice so far, so I will add only one thing:
Keep trying to model a balanced approach to this with your son. Keep the lines of communication open and don’t take a side either way. It sounds like you are doing this, but keep doing it.
Talk with your SP (or someone close to him he respects and to whom he will listen). I know the issues you might face in doing so, but if you approach it humbly and from a standpoint of concern for him, the Church and the youth (and not asking for any punishment for the YM President), I think he probably will understand and appreciate the feedback – unless he doesn’t know you or doesn’t like you already. If that is the case, talk with a counselor or a high councilor you think would understand (and whom the SP likes and trusts) and ask that person to talk with the SP.