Accounts are not designed to be shared. He should have his own Apple Account, but if the grandson is a young child this may require it be done under Family Sharing. Follow the instructions in this support article to create an Apple ID for your child - Create an Apple ID for your child - Apple Support
Related to that:
Use parental controls on your child's iPhone and iPad - Apple Support
Approve what kids buy with Ask to Buy - Apple Support
https://support.apple.com/guide/iphone/set-up-parental-controls-iph00ba7d632/ios
iPad are not designed to have multiple users, or to be used by somebody who is not the account owner. While conceivably you could set it up for him the way you asked, it will require detailed knowledge of the settings (because you will be working against how Apple has designed things), nor will your grandson be able to take full advantage of Apple's services.