Sharing games xbox is using shared user account or my own account when play games?

I believe that you own the game on your own account right? If so, you will need to home share your game with your son otherwise he won't be able to play it from his own account. You will have to sign into his Xbox with your account, home share it, then he can sign in with his account to benefit from the game. You can do so by following the steps mentioned here => Designate a home Xbox | Xbox Support. Oh and yes, when you home share, you share all your games and subscriptions but not the DLC though.

Please be aware, sharing account information and sharing accounts, abusing the my home xbox feature, could end up in account and device bans from xbox live. This feature is for use on one console within a family and giving your account to someone else, accessing
an account you don't own and transferring game licenses to your console with an account you don't own violate the code of conduct and terms of use. Info about account theft:

https://enforcement.xbox.com/en-us/home/staysafe#topic4You can still play your games when you're logged in to the account that bought them on any console regardless of where your my home xbox licenses are.

Please buy and use your own games and stop sharing accounts immediately.

This problem is most likely happening because he's sharing his account with others. You no longer have access to his home xbox.

Hi there! This is Ronnie, an independent advisor.

Nice to meet you and I hope you are having a great day!

I understand you need help on playing your games.

If I may ask, is your son using the account that owns the games he is trying to play?

Since there was a reset that happened on the console, this will also reset the Home Xbox to share games and this might have been shared before for him to play. Try using the account that owns the games and set it to Home Xbox again so he can play the games again as being shared.

Log in to the account that owns the games and do this:

Press the Xbox button  on your controller to open the guide.

Select Profile & system > Settings > General > Personalization, and then select My home Xbox.

To make this your home Xbox, select Make this my home Xbox.

Once done, the games will be shared again on that console.

...there won't be an issue with logging in at the same time as I will (try!!) not to use it when he is not there

As long as you use your own (and not your son's) account, you can use the console whenever you like.

The only restriction is, that one single account cannot be signed in on two consoles at the same time.

But the Xbox One is a "multi user" capable console, so every family member can (and should have) their own account. And with the console's Home Console sharing, multiple people can play one person's games thanks to the Home Console console sharing the games
to other users of the console.

When you logged into the account did you use your own user name and password? Do you have a game score of zero?? If yes to own user name and password and no to zero gamer score it is not a new account. Only other option would be to start a new xbox account
to use the live code for and that account needs to be made the home account to share the live.