Son is locked out of his iPad

My son was gifted an iPad Mini for his birthday last November. He's 17 and had set it up himself, but he's forgetful. He hasn't used it in maybe 5-6 months and found that he'd fogotten both his Apple ID, the associated password, AND the iPad's PIN.

In frustration, he decided to use iTunes (on his desktop PC) to updatea and restore the device, thinking it would wipe it clean. Obviously, it did not.

He has no access to his iCloud account (he'd never set it up) and although he has a Gmail account, he didn't set up the iCloud account to point toward the Gmail account.

I tried resetting the iCloud password but to do that, I need the Apple ID, which he can't remember. There are no emails that he can reference since he can only access his iCloud emails from the iPad (and he's reset the iPad and is also still locked out).

I tried the other Apple support recommendations...all of the solutions require knowledge of the Apple ID, which he can't remember.

As a last resort, I was hoping the account recovery documentation (How to use account recovery when you can’t reset your Apple ID password - Apple Support) to assist with this, but that process requires knowledge of the Apple ID.

His sister bought him the iPad for his birthday, and we've the receipt for that purchase (purchased from Best Buy). Can it be reset by Apple if we provide that receipt?