Sharing ultimate account across two different consoles

No. Sharing is tied to the Home setting and it's only possible to set 1 console as your Home at a time. Across 3 consoles, you would need an Ultimate subscription on at least 2 accounts.

You can't share across two consoles. The sharing feature is for those using the same console.

If you get gold for one account and install that account on both xbox consoles, set one as 'home xbox' and share gold across two consoles. Its a bit confusing but it works.

You can read more about how to set up Home via this link
https://support.xbox.com/games/game-setup/my-home-xbox

After that your son will share your Gold and Game Pass as well as digital game purchases on your account. If this is your first time getting an Xbox One then I would definitely recommend also signing up for Xbox Rewards via this link https://account.microsoft.com/rewards

You're right, you need to set these two accounts home Xbox on the console that you won't use, the way you said it, and all the features that one account has, except for game discounts, the other console will have, as well as that adding content is automatic