Now my cousin starts to look down on himself and laugh at me behind my back like others. What should I do?

Don't worry about him. My relatives prefer to look down on people, but there is no need to feel inferior because they look down on us. His head is on his head. He wants what he likes. You and your cousin will get married separately in the future. After marriage, they all have their own families. There shouldn't be much contact, no matter what he says.