Problem solved! Apparently there was some sort of bug at work, because in the Contact List Clean-Up it showed that I wasn't on his list, when indeed I was. So we both deleted each other's e-mails, added them again and his name was displayed correctly.
I don't know why this happened in the first place. Maybe he's been messing around with his contacts list.
Nevertheless, it's end of story for this issue.
/ 
