it looks like his AI scripts didn't trigger for some reason. maybe he was charmed first before going hostile?
No he wasn't charmed but I found the issue looking through the files with NI, he was assigned a fighter script (I think, the script makes heavy use of potions, but no spells). I assigned a wizard script (nec9 as he had many necro spells) and edited my save to correct the script and he then fired contingencies, time stop and other nasty spells at me!