I don’t remember my password

1.

Your son can: Check his Apple ID via his email box, see this Apple article: If you forgot your Apple ID

If you forgot your Apple ID email address or phone number - Apple Support                  

OR see -> Having trouble signing in?

https://iforgot.apple.com/password/verify/appleid

See this Apple article: If you forgot your Apple ID password

If you forgot your Apple ID password - Apple Support

Then see this Apple article: What to do after you change your Apple ID or password

What to do after you change your Apple ID or password - Apple Support

2.

Then your son need to read -> this Apple article: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch

What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

And remove the Watch from his account, see this Apple article:

Check your Apple ID device list to see where you're signed in

Check your Apple ID device list to see where you're signed in - Apple Support

Then you can sign in with own your Apple ID.

Try to -> Set up your Apple Watch

Set up your Apple Watch – Apple Support (UK)

Se also -> If you can't pair an Apple Watch with your iPhone

If you can't pair an Apple Watch with your iPhone - Apple Support

If your son can recover the password, he needs to do the steps in the article below.

Watch - What to do before you sell, give away, or trade in your watch, or buy one from someone else.   *