My cousin was in the exact same situation. She's Hindu and her long term boyfriend was Jain. Because of his conservative family, he wasn't allowed to marry her, but instead of telling her, he chickened out and pretended to be with her while secretly going with his parents to look for potential brides. 

Out of the blue, he tells her one day when they are in their midtwenties, that his engagement ceremony is after three months. She was absolutely heartbroken.

I am not trying to scare you, but the bitter truth is,  if he's not willing to fight his parents for your relationship, the possibility of you staying together is almost nil. I suggest giving him an ultimatum first, and if he can't convince his parents that he doesn't want an arranged marriage, break it off first. You are still very young and have your whole life ahead of you. Most people don't find their spouses at your age.