Has your brother tried to create another world to see if the items were still there? If the World is corrupted, unfortunately, there is nothing we can do to assist with recovering it :( ^TK
Unfortunately, I won't be able to provide you any other troubleshootings teps here so you will need to reach out to the support and let them know you're still having issues. ^TK
In this case, you will need to reach out to Xbox Support for assistance in this matter: msft.it/6017bsgx7 as they are the team that handle migrated accounts. ^TK
Have you already reached out through a support ticket? My steps here would be basic troubleshooting and I think you and I know that's a waste of time at this point :/ ^TK
Please visit our bug report site to see if the problem has already been reported: msft.it/6015bsjnb. You can upvote if the issue hasn't been resolved. If you think you've found a new bug, visit our help site here on how to submit a report! msft.it/6016bsjnj ^TK
You won't be able to unmigrate your account but if you reach out to us we can work to assist you! Make sure to inlcude your gamertag/username and transaction ID. msft.it/6017bsbRF ^TK