transfer the settings to a child account

Sorry, but nothing can be transferred between accounts. He would have to start over on the child account in order to enforce those settings.

You can create a new child account for him, but nothing can be transferred between accounts.

Time limits can only be set on child accounts.

If the Microsoft account he is using now is a child account and set up in your family as a child, then you should be able to use parental controls on it without having to set up a new account. But nothing can be transferred between accounts so anything he has done through your Microsoft account cannot be transferred to his.

You can't downgrade or transfer an adult account to a child account

Your best solution would be to make a new child account and use the home setting feature to share games

You can create a child account on your current parental account. You can not transfer games, saved progress etc. You can set the parental account to Home Xbox and the child account would have access to the games but not the saved progress.

It is not Microsofts fault that a child account was not set up.