Go there tomorrow. Do not even stop for breakfast. Bring your court order. Tell the mother that you are leaving with the child. Period. If she gets your son to say he doesn't want to go, make it clear to the mother that he doesn't decide that. If the mother refuses tell her that you will call the police if need be.
If she still refuses, even after the police are at her door (and most don't), have the police fill out a report. You will not need a lawyer, but you will need proof, so you need a police report once you get to court.