Redeemed code to an account that doesn't exist

In this case, you will need to contact our live support Contact us | Xbox Support to check from their end.

You say that your son has this Xbox 360 since 2014. Doesn't that mean that he has an account on that Xbox already? If so, then all that should have happened was to redeem that code on the Xbox 360 while signed into the account he already has. If you
created a different account to redeem the code onto, then he would lose any game progress and/or purchases he had with the pre-existing account.

when you redeemed the code it would tell you what you are redeeming and then ask you to confirm that you want to redeem it if that happened then you wouldn't have been scammed as it would tell you the code was already redeemed or doesn't exist if
you were scammed it could take a while for your items to appear though

The problem is that microsoft actually tells me taht my code was valid, but the in-game cosmetics don't appear in my fortnite account.

Yeah, you just might have to - though I wouldn't buy a whole Xbox for one game. Probably just easier to buy the correct edition via the Mojang website or Microsoft Store to ensure its the correct one. Java, Windows Edition and then Console editions are all separate and aren't cross buy to my knowledge.

damn, i didn't expect that, so i have buy an xbox console to use it?