@vikorr,
It's very difficult with religions and cultures. I have a friend who is younger than you, both study and his parents will not accept her either, it's been 3 years now..
There is a lot to consider. You have to ascertain firstly just how old fashioned they are if they could go against what the culture suggests if they were to and loose friends in the process would they even consider it.. I am betting "no". You are outed .
Then, there is the son who rebelled your boyfriend and obviously, in my opinion, that is what you are holding onto, he did it once, he will do it again but he is adamant that he won't.. He did it once end of story.
4 years is a long time, not a short amount of time, his parents can see that he must be happy but they still haven't budged.
I know that my friend, his parents actually really liked her as a person, thought she was good for him, but it did not alter their mind what so ever. They tried to hide their relationship as well... In the end, why do you not want to feel free, deeply loved, being a part of everything ... You are always going to feel 2nd and always going to feel caged...
"If you love someone set them free, if they come back to you they are yours if they don't they never were"...
Imagine..... The truth is in letting go and meaning it, staying away and sitting back waiting... Be honest, tell it from your heart, you can tell him you love him as well, but that you are important, you want to be free to celebrate everything with him and so, until he is willing.. You are going it alone....