It is a bug. He has teleported or otherwise removed himself to a hidden room near the vault entrance, which can be seen on the map but cannot be accessed without clipping through the wall.
I'm sorry I cannot find the original thread where I saw this discussion, but you can see the room on the map and of course can clip into it on PC to see for yourself. PC users can also of course use the console to move him back into the vault proper.
However those of us on console will simply never see him again, unless they patch him back in for us. :(
Oh and he isn't the only resident of vault 81 that can get stuck in that room. I'm missing 3 named residents in one of my saves, ugh...
Edit: Finally found a reference, though it isn't as detailed as the original I am thinking of it does specify the location better than I remembered:
PC ONLY. ok i know whats happening there is a hidden door (vault 81 holding cell) on the top floor of the resident area at the end next to the toliet inside the wall to the left of it. The npc's are somehow getting in there so open your command console type tcl then go throught the wall and click on the door and type lock 140 then move back into the hallway and type (prid 00153855) (moveto player) two seperate commands without the ( ) also do this with the id of other npc trapped in there ( after opening the door and nothing happens you have to turn noclip of so type tcl again then you should go through