Family Sharing Not Recognising Purchase Apple ID!

Spent a day trying to sort this out , but no luck.

I set up Family sharing with my iCloud ID and my Store/purchasing ID. That went fine!

Then, I added one of my sons who has the same iCloud ID and purchasing ID, and that went OK.

Then, I added my other son who has a different iCloud and purchasing ID and this is where the problem has persisted. When I send him an invitation, he accepts it on his phone - which shows the correct iCloud and purchasing IDs - he gets added to the 'Family', but his purchasing ID shows up as his iCloud ID, so he cannot share any iTunes/App Store content with anyone!

We have removed him and re-invited him several times, and have even tried to change the purchasing ID to be the same as what it shows already in iCloud Settings > Family on his iPhone, but every time it is changed - apparently successfully - it changes back to the iCloud ID!

What on earth is going on?