Contact your local CSEA office and establish child support for your child. Take them all of the information you have on your ex. I.E. Address of home, address of his work, phone number, etc...
Once support is established they will simply garnish it from his check.
Bear in mind though that you have the option at any time to cease the child support order or have it lowered or raised. The ball is in your court.
Please be aware though that even though he may be acting like an idiot right now he may turn around in the furture and child support can devastate a person financially and mentally.