How to transfer adult Xbox account to child account.

It is not possible to convert an adult account to a child account. You would need to create a new child account. There is no way to transfer any of the saves or gamerscore to the new account from the adult account.

No adult accounts cant be made into a child account. Nothing is transferable between accounts either.

Yes, you will be able to setup a child account, but you wont be able to transfer his gamertag though. His Xbox account is probably an adult account because it was attached to your email address, and you cannot change an adult account to a child account.
If you still want to create a child account for him though. Have a look here
https://beta.support.xbox.com/help/Family-Online-Safety/child-accounts/add-family-member-on-xbox-one

Unfortunately you can't change an adult account into a child account and you can transfer items among different accounts. Just make a new child account for your child and associate your account as parental account.

Unfortunately nothing can be transferred between accounts. In order to continue using the achievements, etc. on the original account, your child would have to continue using that account, but it would not be possible to use parental controls since it is
an adult account (adult accounts cannot be converted to child accounts). He would have to start over on the child account.