God has nothing to do with this. We are free moral agents. God would violate the free will he has given your loved one by making him stay or go.
The real question is - Why now has your loved one decided to abandon you and your family?
It sounds like he suffers from what so many people suffer from, especially men, which is an over-attachment to his parents. When a grown man still allows his desire to please his parents override his own happiness in life he is bound for marital failure.
Have him read Matthew 25:38-46 out loud to you. The remind him who it was who stayed by his side during his difficult times. Then ask him who was acting Christ like and who was not. Educate him that Jehovah's Witnesses condemned shunning/disfellowshipping as an unloving, pagan practice up until 1952. Did the Holy Spirit mislead Jehovah's Witnesses for nearly 83 years? Why was something condemned in writing as a pagan practice for 83 years suddenly alright to practice. That would be like saying Christmas is wrong to celebrate for 83 years and then suddenly its alright.
Then ask him to use only the Bible and only the 4 Gospels to prove a person must be one of Jehovah's Witnesses to find favor with God or one must accept the Gospel of Christ and be baptized to find salvation? I will give you a clue - you can't prove the first, but the you can prove the latter.
Ask your loved one a hypothetical - if you parents and siblings all died at today in a car crash would you still want to go back? If he's honest with you he would say, "No." Then ask him if he is leaving you for God or for his family. In the end, we should never marry a person who puts their parents before their spouse.