Sons account can't play game pass games I pay for, why?

I only have the one xbox and I think it automatically is set for Home.

Double check the setting. If it's not set as Home, he won't have access to any of your games.

Make sure you have the console set as your Home console. https://support.xbox.com/en-US/help/hardware-network/console/my-home-xbox

[,,,] But i want to using microsoft balance to pay the next subscription bill for my pc game pass

Hi,

subs and hardware can't be payed with credits from your account. That's why it doesn't appear as payment method.

No, the game should detect the existing installation. However, the other account needs to "install" the game once for their own account, so the license is also added to that user profile.

By the way: You can save money by only buying Game Pass Ultimate for yourself, and share that to your son's console (or vice versa). So you only need to pay for one membership, which gets shared to one console you designate as your (or the subscription owner's)
Home Console. Everyone playing on that Home Console will benefit from the shared Gold and Game Pass Membership.

So if your son doesn't have his own Game Pass, and you are just sharing your Game Pass to your son's Xbox Console, you need to do the following on the PC under your son's account:

Have your son sign in with his own account on the PC. Start the Xbox Beta App, and sign your son out, and sign yourself in. Install the game again from your account under his profile. The game should just install a license, and not need to download the full
game.

After that, sign out of the Xbox Beta App, and let your son play the game under his account.