It's a very difficult situation, both for you and him. You need to determine if he still believes, deep in his heart, that the Jehovah's Witnesses are the only true religion. If he does, then the outcome may not be what you want. Either he will stay with you and feel guilty, or he will go back to the religion. If he goes back and you two are married, he will not have to leave you, but being a non JW married to an active Jehovah's Witness is not conducive to a great marriage. How would you raise your children? He would feel obligated to try and teach them about his religion. How would you handle holidays and birthdays? It's a lot for you to take on. If he goes back before you get married, he will be pressured to leave you, they discourage marriage to unbelievers (non JW).
If he isn't sure or doesn't believe this is the only true religion then you might have hope. Encourage him to learn about the organization, there is a lot about their history and practices that they don't tell their members. They also tell them not to trust any information not put out by the Watchtower, so it's sometimes hard for JW's that leave, they are conditioned to fear information from former members, they call them apostates. If he is open to outside information, have him look at JWfacts.com.
Good luck