How can a 17 yr. go back to living with his dad?

mom wanted to get out of paying child support, she lied on numerous things made him feel guilty to the point where he did go to court and told judge he wanted to live with his mom but since then he has had a change of heart and wants to go back with his dad. Mom has also now gotten physically abusive to the point where children services was called in but would not talk to biological dad. Mom has been in and out of his life at her convienence. Change of custody occurred May 30th, 2007.How can a 17 yr. go back to living with his dad?How can he go back with his dad if they cant even get in touch with him! Put the kid in a safe place and make the mother pay child support.