It's possible he read your message with an app that doesn't support read receipts, such as the Gmail. Since the feature requires (voluntary) action of the recipient, even if their email program supports it, it's an unreliable method of tracking email. Further discussion and alternatives [https://productforums.google.com/forum/#!topic/gmail/Irx-gRbIZhU;context-place=topicsearchin/gmail/category$3Aask-a-how-to-question%7Csort:relevance%7Cspell:false here].