Ex husbands possible immoral behaviour with teenage girl.....advice needed?

If it is legal there is not much you can do. It is important that your husband knows, so don't be afraid to bring it up. Just say, look I have something to speak to you about. Then tell him your son was told by this girl that they have engaged in a sexual practice. Let your X know that you are aware that no law has been broken, but it is an uneasy situation since your son is friends with the girl and now he does not want to stay with him. I would not make any judgements. Don't tell him you feel it is distasteful or immoral. Tell him, that perhaps he should speak with your son about it.