My ex-husband has custoday of our son and lives in Kentucky. I live in Texas. For the past year, I have been flying my son to visit me using my friend's airline family benefits. It is a space available flight. My ex is now angry about an entirely different issue and is demanding that I buy full price round trip tickets this summer or he will not put my son on the plane. Can he demand this when he has shown a willingness to cooperate in the past?