Minecraft PS4 created an account by itself.

Unfortunately creating a new Microsoft account with the old email doesn’t work :c

The other option is create a new PSN account and link this new Microsoft account
You can still play Minecraft if your PS4 is the primary PS4

Hello, In order to fix this, you'll need to find the Microsoft account that you used the first time on Minecraft for PS4, it does not matter if you create a new Microsoft account because is the PS4 account the one that's linked to a specific Microsoft
account. If you don't remember which Microsoft account you used, you can go to Mojang support for them to check what's the Microsoft account that's linked to your PS4 account for Minecraft.

Je n arrive pas a connecter mon compte Microsoft sur minecraft Ps4 alors que toute kes mise jour sont faite et que j ai crée un compte Microsoft

[Translation-I can't connect my Microsoft account to minecraft Ps4 while any update kes are done and I've created a Microsoft account ]

Meilleur mise jour MINECRAFT

No thara a ps4 thing and requires a paid psn plus thats a sony thing

https://help.minecraft.net/hc/en-us/articles/360043505132-Creating-an-Account-for-Minecraft-Dungeons

It would be best to create his own account. You can create an offline account on the 360, but it would be best to go ahead and create an online account for him so he can play online now or in the future and can move to the newer consoles with that account.
The account itself is free, but you do need a Gold subscription in order to play online.

Also, while Minecraft is available on the 360, Roblox is not.