If he's behind on the flop, then he doesn't have the odds to chase the last two sixes: calling the flop with the intention of c/f the turn UI is bad.

I fold this preflop. Calling is horrible. You simply don't want to play sixes in a raised three-way pot (which was your intention).

The only other option is reraising to knock out the blinds, get it heads up with position, and take control of the hand. You're pushing a slim advantage over overcards, and you might be getting slaughtered by a better pair, so it's an easy laydown. Just let this go, and wait for better spots.