sometimes when i have a good stack and i have been playing rather aggressively, and i pick up a playable but not great hand in mp/late mp, i will limp to mix it up and possibly throw opponents off. this is what villain could be doing here.

so i think his range his pretty wide. something like 55 or 66 is likely. a flush is possible. i think we would have heard from a big J or a set earlier.

a overly-slowplayed KK or AA that froze up once the turn flush came is also a distinct possibility.

given these, i would value bet the river for t1200, and if he raises...well, i would vomit all over the computer and, assuming it still functioned, probably call.