My grandson forgot he iCloud ID

See if you can recover the ID/password. Use every email address he can think of.

Apple ID - How to find your Apple  -  An iCloud e-mail address is an Apple ID.

Apple ID - Find  -       

If he forgot the Apple ID (iCloud) password, enter the Apple (iCloud) ID, and click Forgot Password. See if he can get a Rescue or Notification email option if necessary. Link to contact Apple Support at the end of the 2nd article.

Apple ID - Manage  -     

Apple ID – Forgot Password  *      

This may help https://support.apple.com/en-us/HT201487