If you check back turn he shouldn't expect you to bet the river so if he has the nut flush typically he will bet it, him planning to and following through with a c/r doesn't make sense.
Given that you have the Q
and J
it cuts down on the probability that he has a playable hand with the K
and another
because typically players will go for high card strength over card suitedness so that K
A
J
9
is in his range more often than K
6
8
5
Definitely looks bluffy, call for sure.