Sharing Xbox Gold

I have two Series X Consoles, one for me and one for my son. I have signed into his Xbox and set that as the home xbox so he can use my ultimate subscription and access my games, however when I sign into my console and try to play a game it says "you're already playing on Xbox Series X you can only play 1 game at a time". If I sign out of his console my Ultimate benefits no longer work for him (can't access multiplayer games etc).

Is this a bug or am i doing something wrong? I thought subscriptions and games could be shared using the above method but it seems like I need to be signed in to his console for him to use my subscriptions, which means I can't play games on my console whilst I'm signed into his!

I don't know if it makes a difference but his account is linked to mine as a "family member" - would it maybe work if I removed him from my family and just let him have his own standalone account?

Any help greatly appreciated, I shouldn't have to buy another Ultimate / Gold subscription should I?

:)