First of all, her parents' attitude is understandable. Any child is excellent in the eyes of parents. They want their children to have a good wife from a normal point of view, but according to the traditional point of view, you don't belong (sorry), so they are firmly opposed. It's not you and them who are against it, it's the married people who are their daughters-in-law.
Secondly, happiness is to be won by ourselves. I think your boyfriend loves you very much. Such a person should be entrusted for life. I suggest that you can't give up, or you will regret it later.
Third, so, if you are brave enough to meet his parents, you don't have to think about what his parents will say. What matters is what you say. Just say that you love your girlfriend very much, and you will treat him well, care about him and give him a happy life in the future. Next, it depends on their attitude. If his parents still object, it's their decision and your boyfriend's. If his parents do what his son says, you must be kind to your boyfriend in the future, because he has done something for you. If his parents think of you in the future, even if their attitude is very bad, you should try to bear it, because you have achieved the happiness you want, not to mention that his parents can't live with you for a lifetime.
Fourth, there is another question you should consider now, that is, whether to have children with him in the future. Of course, you need to discuss this problem with him.