father alienation
Onederland, your son sounds like a gutsy kid! Good for him, for basically calling his father on this. You might want to make sure that your son has a couple of alternative plans worked out for how to handle possible scenarios that could ensue. If he has a cell phone, you might want to make sure it's charged and he has it with him. If not, he should have some other way of contacting you if he needs to - make sure he has your phone numbers and pocket change in case he needs any. Your daughter should probably also know what her roll is, in case she is put in the middle of it. It's not fair, but you won't be there to intervene, so the best thing you can do for both of them is to make sure they both know what it is reasonable to expect from each other. If your son expects more, in the way of support, than your daughter is able or willing to provide, you need to know that, and sort it out, ahead of time.
Good luck to all of you. Your ex has put you all in a thoroughly untenable position.