my dad don't remember his password, what can I do to restore his ipad.?

Passcode or Password?

How to reset your iPhone, iPad or iPod touch if you've forgotten your passcode

How to reset your iPhone, iPad or iPod touch if you've forgotten your passcode – Apple Support (UK)

He can try ⤵️

See -> If you can't connect or sign in to iCloud

If you can't connect or sign in to iCloud - Apple Support

Has he forgotten his Apple ID?, 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

Has he forgotten his password? -> 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

Read again if his Apple ID is locked or disabled -> Related Article -> If your Apple ID is locked or disabled