Before you reset
Back up anything you want to keep.
Sign out of services you use for work, such as Microsoft 365/Office, OneDrive, your browser (to stop syncing), and any other apps tied to your accounts.
Remove the device from your Microsoft account. Go to account.microsoft.com/devices, find this Surface, and choose to remove it. This also turns off "Find my device" for you.
Turn off BitLocker/Device encryption only if needed. Usually you can skip this, as the reset handles it. Make sure you have your Microsoft account password in case it asks for recovery details.
Plug the device into power, because the reset can take a while.
Resetting Windows
Open Settings > System > Recovery (on Windows 11) or Settings > Update & Security > Recovery (on Windows 10).
Under "Reset this PC," click Reset PC (or "Get started").
Choose Remove everything.
Choose Local reinstall (quicker and works without internet) or Cloud download (downloads a fresh copy of Windows, useful if the system has had problems).
On the "Additional settings" screen, make sure Clean data is turned on (it might say "Change settings" first). This overwrites the drive so your old files can't easily be recovered. It takes longer, but it's the right choice when passing a device to someone else.
Confirm and let it run. The Surface will restart several times.
When the setup screen appears, your son can sign in with his own Microsoft account