Does Child need to be in State to expunge his record?

He was arrested when he was 15 in loris south carolina high school. we went to court and we had to do a j rubin long tour . and after that i was sent a letter about expunging his record when he reaches 18. he is trying to tell me that at the age of 16 he has the right to make his own decisions and he can move out. when i was ordered by a court in a different state where i use to live with him i have soul full legal custody of him till a certain age. so what do i do cause we came back up to the other state for his sisters graduation and he is kinda attempting to stay with some relatives who don't have say so over him and they are encouraging him that it is ok.