Read some of the threads of people who are not witnesses married to witnesses; it's a very tough road and one I suspect none of them would recommend going into. Dubdom for kids, I think most on this board would agree, is hell.
Your answers do not sound optimistic. He will HAVE to try to convert you both to maintain good standing with his active family and the congregation, AND because if he truly loves you, he believes he must because those that do not believe will be destroyed at Armageddon which they believe is coming any day now. If he loves you, he doesn't want you destroyed; it sounds logical until you realize how wrong it is.
You should also realize that in dubdom, the man is the supreme ruler of the household and that according to the group, your opinion doesn't count; even more true if you are not in the "troof."
It sounds like he is young and (man I hate it when I make sexist comments) and typically male. Right now he's tasting the worldly forbidden fruit and liking it, but given how little of his world he has even exposed to you it is pretty clear looking in from the outside at your relationship that he is not serious. (I say this based also on 45 years of watching people and relationships and some patterns are so easy to spot).
Remember that love isn't rational, and if you feel it, it is real and that he doesn't have to be a bad person to be the wrong one. If he is truly an active and dedicated dub, you are not wife material for him. He knows it, and what little interaction you've had with his mother is added proof. For him to love you would require him to turn away from everyone else he loves, and if he truly believes that they hold the "troof" then he is not going to walk away from that easily.
Don't let him convince you it isn't a cult; it is. A quick study on what defines a cult will point this out to you, but they are brainwashed to shut down completely when anyone challenges the doctrine. That's why it is so hard to pull someone out.
When you talk, you need to ask the painful questions; how serious is he about his religion? Where does he see his life going? and the really scary one, where does he see you fitting in?
At 20 he may or may not be ready to consider marriage, but he probably is old enough to know if he wants in or out of the dubs. (though this may change at some later date). Don't let him waffle on you. It is entirely possible he doesn't want to lose you either, but I will tell you that if you want more from him, or from anyone else, you will have to ask for it, and at times insist to get it. If you get something wishy-washy like "I want you and them too" or "I don't know," the most protective thing you can do for your own heart is to walk away and suggest that perhaps he should look you up when he knows what he wants.
As far as considering conversion, it is my opinion that no one should change religions to please another, but rather only because it is truly right for them; but in this case, this is no religion, it is a mind-controlling, domineering and abusive cult that will intrude upon and control every aspect of your life. If you convert, they will slowly encourage you to cut off everyone you know and love who isn't "in" and demand every available second of your time "to spread the word." They will promise you eternal life if you obey blindly, never question and do everything they do perfectly. Except that humans aren't perfect and thus you will perpetually be reminded of your failings.
I once posted on this site a list of warning signs that you might be in an abusive relationship that I pulled off the Oprah website (written by an expert); all but a few of the warning signs applied to the Jehovah's Witnesses.
I know it is so easy to say when you are an outsider looking in (to a relationship) and that it isn't my heart at risk, but as someone who has daughter's your age, I'd say throw this fishy back and find you one that is under his own control. You are far too young to commit your heart and possibly your life to someone who will always place you second (or lower) in his life.
I wish you well and heartache on nobody.
