Change my son's account to the child (currently the parent) under a new parent

Unfortunately it's not currently possible to change an adult account to a child account If you wish your son to have a child account then you'd need to set him up a new child account with the relevant settings. You may be able to restrict some of the
features on a parent account but not the full suite that child accounts can restrict.

Since you have a child account (under 21), maybe create a new (or use your parents) account and link your account with your parents account. then you can change settings

well under the parental email

goto manage child accounts

remove it

then add it again under the new account

all child accounts must have a parental account till it matures

Hi. My name is Brilliant, an independent advisor. I am happy to assist you today.

Parent and child accounts on Xbox are designed to help parents manage their children’s gaming activities. A parent account is the primary account that can create and manage child accounts. Child accounts are created for children under the age of 18 and are linked to the parent account. The parent account can set up parental controls, such as screen time limits, content restrictions, and spending limits, for each child account.

You cannot change your account to be a child account. You will have to create a new account for your child.

To create a child account, the parent account must first create a family group. The family group can have up to six members, including the parent account. Once the family group is created, the parent account can add a child account to the group. The child account will be linked to the parent account and will have access to the parent account’s Xbox Live Gold subscription and other benefits.

I hope this helps.

Your son is on a child account. Its not a problem. It needs a parent to approve your son having an account by logging in to the parent account linked to your sons child account. If you don't know the parent account linked to the child account you can try
contact support directly at 1-800-4MY-XBOX and they may be able to find the parent account for you.