This is NOT a criticism, but I’d have told him to keep the five.
About ten or fifteen years ago, shortly after the new plastic bills were issued in Canada, I took a taxi from the train station to my home. The fare was, say $30 so I gave him what I thought was $40 and told him to keep the change. He had
been very helpful with my luggage, taking it from the street to my front door. About three weeks later, the doorbell rang and there was the taxi driver. He explained that two of the bills had stuck together and I had actually given him $60. I assume he had spent three weeks wrestling with his conscience. I was dumbstruck, but no so much that I didn’t tell to please keep it.
Congrats on raising a non-asshole, it apparently ain’t easy, given how many assholes are running around out there.)