I'm so glad your Mom was there for support. What a horrible run around they gave you.
Is he paying child support? Does he even have a job? If he isn't paying support (job or not) I hope you will file for it right away. They are his children too, he may think he can just walk out the door and forget his responsibility to them, but actually he can't.
I also would not talk to him anymore either. I hope you can reach out to other organizations that might be able to help as well. There must be an office in your city perhaps that deals with such things. If they have a main number for city information ,that might be a good place to start.
You don't have to go this alone.