First off, wait for the BB instead of posting when you join a table, it gives you a few hands to assess the competition, and maybe make some notes.

Then, next time you pick up A5o at the CO, you can muck it, instead of checking to the button.

Failing that, I'm not sure you can do much to avoid losing all your money here, to be honest. You should've been much more aggressive at both the flop and turn, especially if you put him on a flush draw. Make him pay to see those cards! But, your read is wrong, and he has a weaker 5, so I don't think he's folding, especially since you're a new player at the table, perhaps just trying to stamp your authority. So, really, the money should be in at the turn, the cards flipped over, you smile, you see the river, and you cry. Damn, why didn't I just wait for the BB?