Hi! Please don't post personal information in this forum; it's public and anyone can see it.
I assume you want to add him to the Family plan so you can set screen time limits, apply blocking filters, and monitor tasks. If that's the case, follow the step-by-step instructions on the website below. You must log in with your account first, and then add your child's account.
https://account.microsoft.com/family/windows/coldstart
If you don't want this, please explain in more detail.