Game Pass sharing between 2 xboxs?

The family settings don't actually affect the ability to share.

Sharing is done through the Home setting on a console and you can only have 1 console set as your Home at a time. So for example, if the games/subscription were purchased on your husband's account, he could set the Series S as his Home which would allow your son to play them as well. Your husband would be able to sign in and use the Series X and use the games/subscription because they are tied to his account, but your son would not be able to unless your husband changes his Home to the Series X.

Also yes, the Xbox Series S is capable of anything it's big brother the Series X can do, so it can use Game Pass features.

If you haven't already, start a Microsoft Family (https://family.microsoft.com) and invite your son and your husband to it. That should solve the problem and share Game Pass across the devices.

Because my 2 kids now share ultimate game pass or whatever is called this really isn't much use.

When the older son had his xbox one the 2 boys could share the gold pass and ultimate pass etc but now he has upgraded to x series these features are no longer shared between the 2 kids.

All I want is what they had when they both had xbox ones is to hVe the same now one has upgraded to x series.