I play the same way against a fish. Sucks he showed up with a 6, but come on now. You have TPTK in a 3bet pot, and villain is retarded. He could definitely do this with KQ, KJ, KT, and he sure has a few bluffs in his range. Also, I've seen fish spazz and make terribad plays with like TT-QQ here. Your beat here by AA, 6x.

I hope your checking the flop, not from fear of c/r, but because this is (1) not a great board for your range, and (2) he is calling a cbet with ALL better hands, and would only be folding worse hands to a bet. That is to say, he wouldn't fold any pocket pair, or 3x,6x. And all of the unpaired hands that he is likely to fold if you cbet, well you beat them.