He sounds like me 25 years or so ago. I was never what you'd call a strong JW. Oh, I believed it. I just didn't like living it.
I was young, I had non-JW girlfriends. But I was still an active JW as well as my family. I broke up with one girl I was dating because I had met a JW girl. My thinking was that at some point, I had to get back to being a faithful JW not only in belief, but in action. The JW girl thing fell apart. Eventually I began dating non-JW girls again. I hid it all from my JW family. With one, it was getting serious. Though I didn't know it, and dismissed my girlfriends concerns, it really bothered her that I wouldn't introduce her to my family.
I'm glad I didn't marry her for reasons that have nothing to do with religion, but I thank her because the one thing she did was help me discover that the JWs were a cult. She showed me some taped program about JWs and at the end they listed some books I could order. I ordered Ray Franz's book Crisis of Conscience and immediately decided that I couldn't believe the religion anymore and I'd never go back**. And I faded away permanently.
**Not to say I didn't have to act the part every now and then. Go to their annual big to-do to please my dad, etc. But mentally, I had no visions of ever living the JW life again. No need to eventually settle down with a JW girl.
I have regrets about that first girl I broke up with in favor of the JW one. Wondering what might have been. And so will this guy. He will look back and think what an idiot he was. Chances are he will never buy into it fully and will leave eventually anyway. If he's open to it, get him to read books like the one I did. See if he can be turned away from the cult.
Leaving is an issue. It can mean your whole JW family will turn their back on you if you do it wrong. So it has to be done right. And he may have to act the part of a JW from time to time.
If he can be turned, there's nothing that says he can't marry you. He would be advised against it. And people might voluntarily avoid him. But technically they can't "disfellowship" him. Provided he doesn't admit to living with you or having sex with you before being married. In otherwords, anything they can disfellowship him for or anything that implies he could have done anything. For them, even you staying overnight under the same roof implies you had sex and they could disfellowship him. Whether you did or not.
And you will have to play along too and understand his predicament. His family won't like that you're not a JW. Not that you have to pretend to be one. But, if he doesn't want to see his family or bring you around, don't push it. He's trying to keep his connections to his family and you. And it probably has to start out separate. He has to guage the situation and react the way he thinks is best.
I eventually married a non-JW girl. My dad accepted it (mom died before all of this). My brother married a non-JW girl too. Dad loved seeing us. He loved seeing his grand-kids. Not every JW is like my dad though.
I hope I'm not giving you horrible advice here or false hope. All I know is he sounds a bit like me. And my eyes have been opened for almost 25 years now.
Some people experience a feeling of betrayal when they find out it's not the truth. Because I never really liked living it, I felt a sense of relief. The way you describe him, maybe he will too.
If you think he's worth it, try to get him out of it. If not for the sake of a relationship, for the sake of a friend. But until he's out for good and knows for himself that it's false and never wants to be a part of it, don't get more involved with him.
VERY IMPORTANT! He can't just leave. He has to know it for what it is. He has to reject their teachings. He has to reject the organization. He can't just be lazy about going and not feel like following the rules. Those people often have flip-flops and return to being a JW. He has to know it's BS. Then, and only then, might he be safe to resume a relationship with.
