It is more likely to be a bug and he is in the process of being deleted from the game.
I see two options.
First you could try the EA version of ResetSim .
If that doesn't work, then either of the two following options ought to do the trick.
If you don't have mods, shut down without saving and then hope he's back in the game when you reopen.
If you do have NRAAS Master Controller try to reset sim using master controller. That rebuilds the sim from scratch. I've used it numerous times to get my sims back when this has happened to them.
If that doesn't work, and you have Master Controller you can click on City Hall or on a computer in the house and look for a list of the sims in town. He might have become homeless on his way out of the world. If he is in the list of sims in the world you should be able to click on him in that list and add him back to your household.
Sometimes you just have to play around with various options in MasterController to see if you can get something to work.
If all else fails, you can either shut down without saving or say goodbye to that sim.
I hope you get him back. I think so far I've always got my sim back after a lengthy panic at times.