Family sharing Apple TV

The Apple TV box (4th generation or later) and the Apple TV app on third party devices will allow for multiple accounts on those devices, among which you may switch as needed on that device, for personalization of the viewing experience. It does require existing and valid Apple ID accounts.

On iPhone, iPad, or iPod Touch, you can’t add multiple accounts.

To set up Family Sharing, do so on an iPhone, iPad, iPod Touch, or Mac — before adding multiple accounts to one device.

Set up multiple users on Apple TV - Apple TV box user guide - Apple Support

Add additional Apple ID accounts - Apple TV app user guide - Apple Support

it won’t let him sign in under his id

Does it give an error message? What does it say?

So it will let him sign in, but there appears to be an issue with the sharing.

1. An Apple TV+ subscription is automatically shared among members of the Family Sharing group, if one member has an active Apple TV+ subscription.
2. Similarly, any of the Apple TV app Channels is shareable through Family Sharing, if one member has an active channel subscription.
3. Connected apps may or may not support Family Sharing, as indicated for each provider separately.
4. Other content in the Apple TV app will have other sharing rules. Movie purchases from the iTunes Store on one the the member accounts, can be shared if Purchase Sharing is additionally turned on for the group. Access those shared purchases is done through the Library, then Family Sharing, then group member name, then title. If you can't find your family's shared content - Apple Support. Shared purchases can’t be accessed directly.
5. Notably rentals are not shareable through Family Sharing.
6. Family members with an account for another country or region cannot participate in Family Sharing.

What types of content can I share with my family? - Apple Support