Who should pay for travel in a long-distance relationship between students? ?

My boyfriend and I are both university students and live in different states. When we met for the first time, I offered to split his airfare. We are in stable relationship. He lives with roommates and visiting him would be inconvenient because there is hardly any room in that apartment. He visits me more often because that way we have the privacy of my apartment. Am I expected to split his airfare every time he comes to see me?