You need to tell this individual in no uncertain terms that he is NOT to have any further communication with your son. None. Do this immediately.
Inform your wife of your decision as head of the house.
If any of them even hesitate for an instant to respect your clearly stated direction on this matter, remind them that you are the God appointed head of the house and if any of them disobey you, disrespect you and do anything to circumvent your stated directions the consequences will be immediate and severe.
Say this calmly and firmly. Do not accept any response other than, "Yes, I understand."