I'm seriously struggling to put villain on a range.

He should think that you have air in your range, and it's difficult (but not impossible..) for him to have any air here. As far as bluffs go, I think all he can have is KQo with Ks, but I just can't see that happening a lot and if he was bluffing with that I would expect him to be bet smaller with that since he would just be looking to fold out your air.

It's highly unlikely that he's doing this with something like AKs, since he beats your air and he's way more likely to check/guess than do this.

I guess he can be doing this just about with a hand like KQs/KJs, but a lot of the time I think this is a full house or better. I think 44 is close but I would just about call and call 88/98 and ship AA/99/A9. Any flush is a fold imo.