Family Sharing must child accounts be MAIN Apple ID on their device?

My son has two apple ids. I have added him to family share with one of them but when he tries to accept the share invitation, it says to try again later. Since it just keeps happening, I am wondering if the apple id i share with must be the main apple id that is logged into the device and not a secondary one. I have read through absolutely everything and can't find the answer to this anyway.

So, again. one apple id is main apple id where he can send texts and get email and the family apple icloud id is logged in as a 2nd icloud account on his device. he doesn't want to switch them as the main one is where he wants to use messages. the secondary icloud is the one he has always used for itunes and we want to be able to share what he has purchased there so we want that 2nd account to be the family shared account. any help??