Xbox one with minecraft pre loaded

Consoles do not come with a game pre-loaded. If a console does come with a game there should be a code in the box to redeem. If his console had the game pre-loaded then that means someone else had used the console and left it as their Home console which
would have allowed him to access the game on his account. If that person changed Home consoles, your son would no longer be able to access the game without purchasing it.

No lo trae precargado, viene con el juego

There should be a code on a pamphlet looking piece of paper inside of the xbox packaging

If the game was pre-loaded when you bought the console, then someone had most likely used the console before you and used the code. If it was just recently purchased, you would have to try contacting the retailer you purchased it from for an exchange.

You can also check the accounts purchase history to verify which account owns it. The link for that is here