Hello DaveE123,
I understand why you’d want to keep the progress your son has built up over the years. Microsoft doesn’t offer a supported way to move Xbox saves or game progress from your wife’s account to his new account, even if you repurchase the games.
There may be a separate, unofficial possibility for Minecraft Dungeons characters if those character files are available on a Windows PC, but it would not transfer his Xbox profile or all his game progress. Has he played Dungeons on a Windows PC, or only on Xbox? That will help determine whether there’s anything worth checking beyond the account-transfer limitation.