My exwife is moving to Korea and our child wants to go live with her. Do I need to surrender primary custody?

Currently my child lives with me. His mother and I have join custody but now she is moving to S Korea on a DoD employment. Our child 15 has expressed his interest in going with her. I have no problem my son going out there but His mothers insists to change the primary custody so she can enroll my child in school out there presumably also to receive government allowance. Is there a legal way I can authorize our child to travel for maybe 6 months, live there and be enrolled in a ARMY school without surrender my primary custody? I only want him to try living there before I have to sign a suit to modify document. How can I protect my kid that in case he wants to come back to USA for any reason?