Two options:
Go after him for child support through your local child support recovery office. This will make it "uncomfortable" and maybe he will see the "not paying child support" as a better option.
Or you can file an abandonment charge. This is the route we are currently taking.
I don't know if your ex will have to pay the child support arrearages but I was told by my lawyer that once we have his rights terminated that he will not have to pay child support anymore. The $50 a week isn't worth it anyway.