[Harv72b]

Even though he didn't attack your blind preflop, this still falls under the umbrella of being a blind battle. Your opponent is also pretty aggressive, and this turn is almost ideal for a semibluff raise. It's also very possible that he's just holding something like A6 and thinks it's good.

I would very rarely recommend putting 2 more BBs into a 6.5 BB pot on a board like this one. I think this is one of those times.

Incidentally, the 6 only made him a straight if he's holding a 9 or 45. If he's holding a 9, I think he raises the flop on a semibluff/free card play nearly every time; he might do so with 45 as well. This also leads me to believe that he's (semi)bluffing.