my 12yr old son wants 2 move in with me how do I go about this my X partner has agreed

my 12 yr old son wants 2 move in with me and my X partner has agreed how do I go about this

Were you married? Was there a determination of who would have physical custody of your son in a divorce judgement? If you weren't married, he can just go live with you, if there was a judgement regarding custody, check back with the courts about ammending physical custody.