This is 10NL he doesn't have to do anything. I see people at 25 and 50NL "slowplaying" AA KK AK some times multiway oop like this (although not often). Given that he's an unknwon and this is 10NL, I think it's far more likely he's doing this than bluffing. Do you think he's trying to bluff you off your preflop holding after you've just called off 20BBs in a protected potl? That's a far more ludicrous proposal in my book then him being a tard with a premium hand. If he doesn't have a nut range like [AA-QQ AK] he's likely back raising here with stuff like 99-JJ AQ that he just can't bring himself to fold and doesn't known what else to do with. You don't actually fair much better against at this range at all, it's still the same proportion of overpairs to overcards.

You have 88 and have to at least have seen him take retarded bluffy spewbag lines that noone ever really takes with air before you can just stick 100bbs in with 88 preflop.

I think if he's being a tard with worse pairs than 88 or 89s and weak stuff like that he's far more likely to just shove and try to make you fold. The fact that this is a protected pot he can never win preflop makes it even more unlikely he's ever going to be bluffing here.

I agree it's a terrible way for him to play AA KK etc, but him having air here makes even less sense unless you know him to be terrible and very spewy preflop. Whatever he has he's bad and bad players are not bound to follow any code of what they may or may not "have" to do.