Can I sign my son up for gold on another console at his moms

You can sign into his account on xbox.com or the Xbox app and and redeem it through there.

I have made one console the home box...I have the second completely set up. But....it won't allow live? ...

Very frustrated Momma

Is it possible that you bought Xbox Live on your parent account, and your two kids have their own accounts?

Be aware, that your account can only share to ONE home console at any given time, and you can only change the sharing to a specific console five times within a year! So be careful when you try switching around the Home Console!

I'll assume that you bought Xbox Live Gold on your own "mom-account". With two consoles, this will only work if you set one console as your Home Console, and are actually signed in together with your second kid on the second console.

Ideally, I'd recommend the following:

Have Son #1 buy a Gold membership after you cancelled the Gold membership on your "mom-account". Then have Son#1 set console#2 as their Home Console, so the Gold membership is shared to Console#2. Now everyone on console#2 can play online multiplayer.

Son#1 can go to any other console and also play online multiplayer, as he has the Gold membership tied to his own account, which roams to whatever console they go to.

Also have one son buy the games that both kids should be able to play, as the same sharing rules apply: You with your mom-account can only share to one console at the same time.

When son#1 buys a game, the ownership of the game will roam with son#1 to whatever console they sign in, while his games are shared to his Home console that son#2 uses.

No Problem, set the XBOX One at his Mom´s house as your Home Console. Than can your son use your Gold Account

And this won't remove the subscription from my son's console, right?

Subscriptions are tied to the ACCOUNT that bought the subscription.

So your son can go to any console in the world, sign in there, and he will have access to his Gold membership subscription, Game Pass (if applicable) and digital games.

As long as he's signed in on a specific console, that console will also share the Gold membership to anyone playing with him on that console. Even if your son is not actively playing, as long as he is signed online (for example on a second controller), another
person can play on that console and access his games and benefit from the Gold membership.

The account that owns the subscription, can designate one console as their "Home Console" (or as you call it the "Main console") and will share the Gold to that CONSOLE. Any previous "Home Console" will lose the ability to share Gold and games when your
son designates a new Home Console.

Your son's account will still keep his Gold membership, and will share it to the designated Home Console, which in turn allows anyone on that console to benefit from your son's games and Subscriptions.

So is there a specific reason why you want to keep your son's console the Home Console? Are you (or someone else besides your son) playing on that console and needs the subscription?

If so, then there is no other way around than to buy a separate Gold membership for your girlfriends kid. Or you'll have to play on that console to benefit from the shared Gold membership after setting the "girlfriend console" as your son's Home Console.

Thanks for this.

The only real reason I wanted to keep the 'Home Console' designated to my son's Xbox, is that I'm not very clued up on all of this, and I was concerned he'd lose his access to Live Gold.

Yes. Once your oldest son signs into the younger son's console and sets it as his Home, the younger son can access any games on the older son's account as well a gold subscription if he has one. Then he can sign out and sign back in on his own console
and still access the games while the younger son signs in and plays on his own console.