Lost access to my son's Apple account

I lost access to my son's apple account who is in my Family because it's trying to send me a code to my old phone number which is no longer active. When I try to go through the "I can't use this phone number" and choose another way to authenticate, it goes back again to asking me for a code sent to the old phone number! What to do?? Help! I've been trying to do this for two days now!