Tough decision... I'm thinking that you need to bet here though. Probably somewhere in the $1.25 to Pot size. If he calls, then you can probably narrow his range to Ax or flush draw. Some players at 5nl won't chase the flush with one card to go though so betting out may get him off the hand.

If it's Ax, and it's not A-J or A-Q, he probably just calls here. If he calls and then checks the river, I'd check behind unless I hit the flush myself. I've run into a lot of players at 5nl who will c/c with two pair and then push the river or c/r me all-in if I bet.