Personally I call and then see what he does on the turn/ hopefully you improve. You have two pairs / blockers and you have an open ended straight giving you a lot of showdown value. Even though you haven't given us any info regarding the villain we can still go through the possibilities. If he has trips in this spot (which seems like the story hes trying to tell us here because he can't have an over pair and reraising with Kings would be dumb obviously :b). So that said it must be trips because you block the higher straight quite nice and it would be ridiculous for him to be going crazy with 34 or 23. Seeing as you block the A and the 6 he could have a set of 5s possibly. That said if he does have the set its a race at this point. So really it comes down to your read I think. Is he a LAG or TAG? If hes a lag and I've seen him try to make plays like this then I'd probably call if hes a tag then i might have to think it over for awhile before I make the fold. There is also the possibility that he saw a lot of chips in the middle along with an untextured board and went for it. All that I know is that if he does have the trips its a gamble on the flop because your EV is only ahead by a fraction of a percent.

If we knew what kind of player he was and had some stats I think it would be easier to say with more certainty. Also, an HH is very helpful too :D