HI there,
Ive had a simular situation. But my son was 18 at the time. His father was abusive and was under mental health. It broke my heart, but my advice to you is to let him go and see his father. A lot of the time, people dont change. Your son will soon see the warning signs and will be back where he belongs. Your son may just want to ease his curiousity and if he does not go, it may still make his GCSE's suffer. Just explain to him that you will be there and that he has your blessings. I understand fully how hurtful this can be. Just dont say I told you so when he does come back. He will need to feel that he can come back and admit he made the wrong decision. It took my son 8 months to realise. He found it hard to admit that he made the wrong choice because he was afraid I would say I told you so. But now he has no contact what so ever. Any of your adult family members or yourself should be able to be with him at the time if you feel he may still be abusive.Wish you luck.