Can't add main account after 3 share

Contact official support to see what has happened.

Well I did that. After a LONG time on the phone, the support person advised that the Expansion Pass does not get shared with other users on the same Console. Now I'm the parent, so I do the buying, on my account. The main game (long ago) and this Expansion
pass (recently) were bought by me. But when my son plays on his account, he sees the main game, but cannot access the Blizzard Mountain add on, though he can access the Hot Wheels add-on (both being provided by the Expansion Pass). I can hardly believe
that. The main Horizon 3 game does share to my sons accounts on the console.
Isn't it reasonable to expect that all add-ons are shared unless it is stated otherwise in the Purchase Terms?
How is anyone to know? Does a dad with 3 kids have to buy the add-on 4 times? And when do you discover that? After you've bought it once and it doesn't work for the kids?

You need to setup your console as home console

Once you do you can use family share and add him as a guest account on your family share

Did you add gold to your main account?

Contact official support to see what has happened.

Well I did that. After a LONG time on the phone, the support person advised that the Expansion Pass does not get shared with other users on the same Console. Now I'm the parent, so I do the buying, on my account. The main game (long ago) and this Expansion
pass (recently) were bought by me. But when my son plays on his account, he sees the main game, but cannot access the Blizzard Mountain add on, though he can access the Hot Wheels add-on (both being provided by the Expansion Pass). I can hardly believe
that. The main Horizon 3 game does share to my sons accounts on the console.
Isn't it reasonable to expect that all add-ons are shared unless it is stated otherwise in the Purchase Terms?
How is anyone to know? Does a dad with 3 kids have to buy the add-on 4 times? And when do you discover that? After you've bought it once and it doesn't work for the kids?

UPDATE: Accepting the above advice form the support person, we asked Microsoft to cancel the order on my (Parent) account. They did. We then re-ordered the Expansion pass on my son's account. Guess what? Behaviour is exactly the
same when he plays. Hot Wheels is playable, but Blizzard Mountain is not, and attempts to "go there" are met with an invitation to purchase something. I note also that in the "Manage Apps" screen, I can see a "Forza Horizon 3 Expansion Pass" (10MB), a
"Forza Horizon 3 Hot Wheels" (2.4GB) but there is no "Blizzard Mountain". Could there be something wrong with the download of the Expansion Pass (which is supposed to provide both Blizzard Mountain and Hot Wheels)??

So you have one main account witch you pay for then you can add more accounts witch are free but can use everything the main account can is that right? Is this done on one XboxOne or is it based on a home meaning you could have multiple xboxs witch can
share everything from the main account.

You can't completely delete an account. You could just add live to your main account.