Maybe you should try getting to the root of the problem. So many times when there is a kind of feud, let's say, most people can't even remember what even started it because it was so minuscule; after it is discussed, people get over it and move on. I agree that you should certainly be supportive, but try to encourage him to at least learn (or tell you) why his family doesn't support your relationship. Maybe it was a misunderstanding, which can be fixed. Maybe it's a ridiculous superficial reason, which means there's nothing you can do to change their mind but you should continue to be the good person you are. In the end, at least knowing why they are the way they are will help you figure out next steps as he is your family for life and you don't want to be in this uncomfortable situation forever.