What I did in a similar phase that worked, (after trying what you are doing) was sending emails with questions, that required yes or no asnwers. Like "this weekend I need to work, can you take care of them or should I find a babysitter? Kids need to be driven to xyz, will you do it?" etc etc
I had answers to my practical problems, record of his behaviour and gave him time to think about it each time. I wasnt making it an US problem, it was "things that need to be done that have to do with your kids". Clear, firm, realistic and with no emotions. Think about it for the future...
