What expenses is child support supposed to cover?

I pay child support to my children’s father because he has primary custody of them due to me traveling frequently for work. I am always on time with payments and owe no back support. The problem is that he is constantly asking me for additional money for various things, like their school trips, clothes, fees for sports. He says what I pay in child support doesn’t cover these extra things, it only covers the basics like housing, food, and health insurance. I thought child support was supposed to take care of all of their expenses when they are with him. He says if I don’t contribute to the extra expenses he’ll just seek a higher child support payment. What is child support actually supposed to cover and can he get more support if he takes me back to court?