The reason I said it was the son's fault is that:
1) He's seen his sister do this to his mom, and she drops everything and helps. This is learned behavior.
2) He's old enough that he knows what his own social agenda is. Like her daughter did before, he waited to call mom until the last minute, even though he of course knew what time he was supposed to be at the party. It's a safer bet that he's going to the party if he calls mom at the last minute, than if he asks his dad way ahead of time.
I've read your posts before, and your children are well aware of what's going on between you and your ex. Not only do they know it, but they play both sides when it suits them. I do not feel that putting a child in the middle is the best bet, BUT when this has happened before, and is a known pattern of behavior on behalf of the children AND the ex then she needs to put her foot down. Her ex has taught their children to play this game well, and it isn't going to stop unless ground rules are set down.
And you were supposed to buy a present for this party too? If your son knew he had to buy a present, it meant that the ex knew he needed to buy a present. And didn't. And left you hung out to dry yet again. Knowing their father's feelings for you the kids in this situation need to learn to communicate with mom more. It's not that she doesn't want to help, it's that she's tired of being thrown into it at the last moment.
Ex's can be civil when need be. I kept our kids on his time this past week when all three were sick. I missed work, my husband missed work, and the ex only missed one day. Ex was peaches and cream when he needed help, but was an a## the week before. He acted this way because he knew he needed to be "normal" to gain my assistance. I got "normal" for a week while they were sick, but it'll revert and I don't care anymore. My kids get disappointed when they wait til the last minute with me, and they know it. I don't have that problem now. In a true emergency when they're ill, then yes I'll drop what I'm doing to help, but a dance or a party is not an emergency.
When your kids start holding EX accountable instead of YOU, then you'll know you're making strides in the right direction. It's not that you want them peeved at him, it's that you want them to grow to be mature adults and they NEED to see your ex do the same. For their own sake--when they're parents.
Somehow you're going to have to retrain your ex to pick up the phone and call you. And since you have such a poor relationship with him (more like he does with you, but we both know how THAT goes), the kids will have to modify how they handle things until the ex can act like the adult he should be. Hence, your son knows better by now.
FLMom