We discovered the issue was his Topics were set to Tracking not Watching, thus, if I replied to the topic (and not directly to him), he wouldn’t get a blue dot notification, just an unread counter on the topic itself. 
Ah yes, the distinction between the two isn’t exactly clear until it becomes clear through experience. At least that’s how it was for me.
Slightly annoying considering how the old Sitepoint Forums were set up, but alas it’s fixed for me now thanks to @cpradio