I've just now noticed the missing diamond on his profile, along with this:
There's no announcement on Meta, and this just seems like an abrupt change. I'm not sure how long ago this was, either. But I probably should've suspected something when it seemed to take a tad longer for my recent moderator flags to be reviewed.
I don't know too much about the moderator system, but I believe that the Pro Tem moderators are appointed by SE. Unfortunately, that means that SE is responsible for appointing another mod since we obviously cannot hold an election until graduation. Overall, fewer active diamond mods is not good, even though this is a small (but growing) site. Of course there are the SE employees ("universal" mods), but I don't think they're obligated to moderate Beta sites regularly. Which is why Pro Tem mods are needed.
Could another moderator or anyone else weigh in on this? If necessary, this could be migrated to MSO. I'm just really surprised by this, considering such a change should be quickly known to the community.