Many people here will give you very good advice on what to do if you fall for a Jehovah's witness. My own advice can't beat what others have already said. I want to make a contribution, however, and, from my own experience. From your post, I gather that you're not really clear as to where you stand with him or what is going on.
Oniichan's religion frowns on their having any sort of relationship with "worldly" people other than Bible studies or preaching. What is normal for you and me, like hanging out, watching a movie, having dinner together in that little restaurant around the corner, giving them a simple present, that is not normal or expected from them. He knows that very well, and has always known it. Yet he's a human being and fell for you. In doing that, he's breaking the rules. His mere befriending you, I understand, wouldn't get him disfellowshipped. Neither would his marrying you. However, his seeing you is indeed very strongly discouraged.
I bet he had no trouble relating to you when you had a boyfriend. That was because you were unavailable. This is warped logic, yes, but that's the way it works. If they can't have you, then they are safe with you. The minute you become available, you also become a risk, and it is then that they have to avoid you like the plague. Because you are indeed the plague for them.
He was deceiving himself when he said you couldn't be boyfriend and girlfriend but could be friends. He knows full well that he can't be your friend. But he is a human being and followed his heart.
He does love you. And he isn't doing what he's doing because he's mean. Now that his secret is out, he can't see you without serious consequences. I'm afraid he's too young to be on his own, and probably he knows it. He's also been raised under the belief that he will die forever unless he follows whatever the Watchtower Society tells him to do.
Dandingus says you can either become a witness, or hope that he will be out. Or, accept that nothing can happen.
Me, I would wait and see if he can be out. Or, if he will take the heat and be with you anyways. In my lucid moments I am firmly convinced that no one should join the religion. I don't think joining would be a good option for you. I do urge you to read about the Watchtower as much as you can. This forum would be excellent for you, as it has been for me. Don't dive into the religion without knowing what you would be getting into.
He might leave the religion for you, only to return later. Bear that in mind.
I agree with Dandingus that most likely the guy will stay in his religion and you will have no choice. He knows this full well, because he was in.
I encourage you to read a lot, with patience, asking yourself questions and trying to get answers for them. I know you're in love and, for you, like for Dandingus' girl, leaving might look like too much. It does look like too much for me, you know? And I'm a lot older than you are. You will only reach a decision that you like after a long time, and long thoughts.
I wish you and your Oniichan the best, dearest.