k, I still think he's blustering now and won't actually serve you until he can get his crap together. So you still have the head start and will probably be able to serve him at the apartment. Don't feel bad about having your own L do all the work--you would need to know all the financial stuff no matter what.
I'm so sorry he ruined the day like that. But cheaters are self-centered jerks so it's par for the course. I hope you can get S13 to talk to someone so he can let it all out. How horrible to dump this all on S10 like it's his fault because he wasn't ready to go!
I'm with Pearl on this. Not much else to add.
Except that if everyone could see how cute your little S10 is and how sad he was yesterday and how he thinks it's his fault that Dad left.... and then his texts and calls to stbxH are ignored.... And the S13 is already quiet and withdrawn to begin with... I alternate between tears and furious as hell for them...