When I share my Xbox game pass ultimate, does the other person also get gold and can play...

Find out what kind of error message he's getting when he tries to play online. If it's asking him to buy Gold/Game Pass Ultimate, double check your subscription as well and make sure it is Ultimate.

Cloud gaming is only available or the person who purchases the subscription, so that's normal, but he should be able to play online through sharing.

Thanks for the quick answer. So that's for cloud but that's fine.

Any idea how I can solve the problem ? Is there a setting for that or is there a way to enable gold for him?

Have him try doing a hard reset (hold down the power button until the console turns off, unplug the console, plug it back in, then turn it on).

It's definitely ultimate, just checked it, and yes it shows him the subscription menu

I'll tell him that when he's back home, I'll text later if it worked or not, thanks