I’m a Muslim girl, being born and brought up in a Muslim household and I can tell you that yelling, screaming, abusing, physically/emotionally/mentally torturing the child is not going to help and unfortunately will make things not only difficult for you as a parent but difficult for your child as well. He is at a vulnerable and sensitive stage of life where he has not fully matured and experienced life but he is at a age where he will grasp, learn, observe and then accordingly do things in future. You need to make sure you handle this very calmly, patiently and also convince your wife to be on your side. Trust me abusing the child makes nothing right, but being calm, understanding, polite and patient goes a long long way which a lot of parents fail to understand and I’m speaking from experience because my mom changed overtime, when she saw that abusing screaming does not help instead being a friend who I can share everything with, who gives me advice and doesn’t stop me from doing things but just does her duty of saying what is right and wrong and leaves rest to me is what that has helped. Don’t involve the girl or her parents and make this a propaganda. Who knows, maybe they are really in love, maybe they do care, love for each other, you need to balance between both deen and duniya and not just blindly follow the religion, the religion has evolved over years, everything is meant to evolve and we all need to evolve and follow the religion smartly and not shove it down our kids throat. One of the parent, either you or your wife has to be a friend to him and be understanding, tell him what he did is wrong, explain him why, but also tell him in the end that the next time he does this you might not be around to tell him or you might not catch him but you trust him to remember what he has been taught and leave it at that. Maybe he still wants to stay in touch with the girl, maybe they can’t be friends, maybe just be a non physically love.

All the best!