Pay the bill. It’s a legitimate bill.
Just because he has a paper diploma doesn’t mean that any request for a transcript will be provided. You don’t want that looming in the future!
My dd recently got a bill for leaving her bike on campus. She told us, “its old and rusted and I don’t need it”. We told her she’d be “old and rusted and not needed” by the time the school would choose to send out her transcripts for grad school.