Well first of all, don't tell them your dating. But if they find out, say that he is in our religion (or whatever you believe in). You can add more things to it. My Boyfriends parents didn't like me at first. But they knew that since i've been dating him for about 4 months now, they trust me.
ask your parents why they dislike him
try and convince them into liking him, tell them about the good things about him instead of the bad.