There is no straight answer to this, imo. Your BF needs to have the cojones to make some kind of decision here. Whether he wants to marry you without telling his parents or not. I dont think his parents will be receptive to the idea but i am sure they will gradually come along.
I did it without telling my parents, and i am sure my mother was hurt a bit in the beginning but later on they accepted my decision. But my relation did not work out because of religious reasons and we had to go different ways.
She was not very religious but since one of her parent was a preacher she had some kind of roots in the subject. I even went to the church in the beginning couple of times, just out of curiosity, mind you, i would never change my religion for any one or any reason. I am not very religious but i believe in my faith strongly. Then we got married, both of us were going to college and she changed her major out of no where to Religious Studies and decided to become minister at a church like one of her parent. This took me by surprise and i was fine and it was ok for a while and she never forced me her religion neither did i force mine on her.
But this had immense repercussions on my schedule as i had to help her in her project and accompany her to various functions and venues which i could not shoulder any more. And that was that.
In my retrospect this subject in not a cut and dry situation like many believe. Later on i backed off another potential relationship where the person i was talking for few weeks was pretty religious. She would travel to different states for prayers and conventions and i was like WTF. I am not saying i would not go outside of my culture or religion in finding my partner but i would not be seeking devout religious individual.
But this should not discourage you at all. Hey you never know until you commit. Best of Luck.