I don't know. Try the console command 'getav health' to see what his health is. Maybe he has an insane amount?
Will do.
MMM or FWE won't cause this.
Mods like CASM and Responsive Kill reactions(though I think it's fixed in the latest version) could cause this.
Regular quickloading can also cause rare abnormalities.
I'm using CASM and seem to remember something about occasional weirdness after quick loading. This wasn't after quickloading, though, although I do use it.
I'll load from an earlier save and see if it helps.
EDIT: Loading from the save I made just before making the thread he was his killable self again. So it seems to be connected to save game glitching somehow, as he was unkillable from the previous save. I'll use CASM quicksave/quickload more sparingly.