NBTExplorer can fix it.
Open the file name for his flawed world, if mine, 2021Apr03 1.12.2
Near the bottom, you'll find "level.dat", expand that and you'll see "Data ## entries"
Under "+ Version:" near the bottom, you'll find "SpawnX" "SpawnY" and "SpawnZ".
Changing those will change his spawn point.
A couple of cautions:
1. Make a copy of the game as it is on your desktop. (just in case something goes wrong you can overwrite what is under "C:\Users\Hidden(Name)\AppData\Roaming\.minecraft\saves" with the copy you made to the desktop and everything is as it was!