Is it his ego that makes him utterly stupid, or is actually stupid?
I've often wondered this. He will make a claim, which suits whatever argument he's trying to make, then 10 minutes later he's making a contradictory claim. He can't keep a straight story for whatever reason - maybe it's because he lies so much (if you're telling the truth, it's consistent, and you don't need to work at remembering different things you've said). Or maybe he is actually stupid and because he can't remember the stories or see the inconsistencies he assumes no one else can either.
If you ever try to debate him, it's like trying to nail jello to a wall. He's in full JW mode - switching topics, pouncing on the different meaning of individual words and obsessing about the slightest error anyone makes. But completely missing any coherent logical argument.
Personally, I think it's both - it's ego and stupidity.