It is unusual that the father allegedly punched him in the face and did not leave any bruises. You may want to get more details from your son before pursuing any further. After an incident like the one described, you should take pictures of any evidence, such as bruises or red marks, and you should immediately notify CPS and the police. If, after getting more details from your son, you feel that he is in danger of being harmed again, you should seek advice from an attorney on possibly filing for a protective order and, if you have an existing court order for visitation and custody, modifying the visitation.