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.

They actually did not give you correct information. It is not possible to request a gamertag. If it's already in use, you can use the tag, but you'll be given a suffix to differientiate you from the other. Otherwise, a tag is only freed up if the owner changes it to something else or they don't sign into their Xbox account for 5 years. If a person doesn't sign into their Microsoft account within 2 years, that will close the account which should also free up the tag.