My gut reaction here is that he has a pair and a flush draw. Not sure what other hand plays like this. A made baby flush probably leads out for the pot and a nut flush slow-plays. Any other strong hand that is not a flush has to lead out with a strong bet. However, at these stakes on Party, this guy could have any 2. Either way, I think the call is very questionable since your either a small fav or huge dog. I'd probably fold and look to get my money in the middle in a better situation.