How to correct game tag from xbox app to actual minecraft account?

Thanks for the quick reply.

He actually has a child account, but his 'gamer tag' is associated with my account. From what I have researched, you cannot move a gamer tag to a child account on an Xbox One. If that is not correct, any references to how to do it would be greatly appreciated.
If it matters, and I know it does for Xbox 360, I have an Office 365 subscription on my MS account. On a 360, that prevents moving gamer tags.

When trying to add the App on my computer, there was a "made up" gamertag given to me by Xbox Live. This tag is not the same tag that is actually on the Xbox... How do I take off the account active online and associate the real gamertag with my online account?

You are correct.

Once you have the new account logged into the Xbox and with Xbox Game Pass activated. Any account on the Xbox will be able to play Xbox Live and Xbox Game Pass games. Your sons account as mentioned by Zenosos will be
able to play his Minecraft worlds as they are not tied to specific Xbox Game Pass subscriptions but to his Gamer Tag.

From your description, it sounds like Minecraft is the only place that is not correctly displaying your gamertag, so you shouldn't try to change your Xbox gamer tag, since it is already correct. It must be another issue. The first thing that I would recommend
is just making sure that you are signed in with the correct account within Minecraft. Additionally, it might just take some time for your name to update within the game, if you just made the change.

I ha e never been asked to log into minecraft as its on the ps4. The remote connect login page doesn't show any gamer tag. Only asks you for your Microsoft login details. Email and password. I've downloaded the xbox app and logged in with the same details
to check and it's the correct account for when I had an xbox. But when using the same details on the remote connect for ps4 minecraft, it's logging me in with a completely different gamer tag and gamer tag picture.

To add to the issue I've logged out of that account on minecraft on the ps4 and tried again using remote connect on the laptop (to see if there's some issue with logging on with my phone) and after entering my email address and password, the ps4 minecraft
says that it will not log in with these details as I've already logged in with the gamer tag I don't recognise. It let's you sign out but it says I can only use that a account.