Cannot share a purchase to my son’s ipad

Hi there, I’ve downloaded the Getty-Dubay handwriting app and bought the in-app purchase of Book B on my ipad. I thought this in app purchase would have been available on my son’s iPad (we have Family sharing set up). However, it is not there. I have been back to the developer and they have tried to help, but it will not show up on his ipad, no matter what I do.

Apple have refunded me the cost of the in app purchase so I have tried to purchase it again directly to my son’s ipad. It goes to ‘Ask Permission’ when I do that it says ‘Purchase failed: the payment is deferred’. I have the correct card details.

Can anyone help please? I have deleted the App, reset it - I don’t know where I can ask Apple directly?! Cheers :-)