heres the problem imo ... if the button is bad enough to call here with the intention of floating a bet into 3 other players, hes definitely bad enough to flat there with a set of 5s, or two pair KJ. theres pretty much no way to tell the difference, i think its just more of a timing/feel decision as to which it is. did he think for a good amount of time before flatting the flop? then probably a bigger hand, if he didnt think much then i think its more likely something weaker. it also makes sense that he woudl slowplay a big hand there in hopes to get another player to tag along .