My grandson clicked find my iphone and now his ipad has reset to brandnew and wants password
his dad doesn't remember his icloud password
An account's password can be reset via http://iforgot.apple.com
If it can't be reset that way : Contact Apple for help with Apple ID account security - Apple Support
Or to remove the iPad from the account, and if you/he were the original purchaser of the iPad (i.e. it wasn’t bought used), you could try going to an Apple Store (make a Genius Bar appointment if you do) or try contacting iTunes Support online (https://www.apple.com/emea/support/itunes/contact.html) and see if they can remove the iPad from the account - they are likely to want to see proof of ownership before they might help e.g. original receipt with the serial number on it