Here's another one, what's fair here? (please see details)

I have written of my familial problems here before- My daughter has been living with me for about one month. My son lives with my ex, very far away. My now husband paid for all the paperwork and one way fare for my daughter to come, about $2100.00, plus she will be going to private school. My ex wrote to me wanting us to pay ½ of my sons’ airfare to come to this country for a visit in June (about $1500.00). My ex says it is to visit our daughter, who by that time will have been gone from his home almost 4 months, and his itinerery includes visiting other cities and friends. It doesn’t sound like I will have much time with my son. We have offered repeatedly to fly my son over for a week to 10 days to visit us here and stay in our home. My ex always said he could only do it if my ex came too, so it never came off. He had no problem with my daughter flying that distance by herself. Now that my ex wants a holiday over here, this has come up. I don’t know how to sort it out. I don’t know what to say to my children without sounding like the dead beat ogre my ex has made me out to be. I don’t know if I can ask my now husband for this money.