what are his aggression stats? any history? I don't see people bluffing very often here, as you can only rep a BDflush - have you seen him bluff in any other spots where he can't rep much? Also, to be bluffing he has to be turning a made hand into a bluff - have you seen him do that before?
i suppose his value range is QQ with diamonds, T97* with diamonds wich seems pretty consistent with his river play. Also turning made hands into bluffs and calling the turn to bluff diamonds and rivers seems pretty advanced and i dont think this fish thinks this way, for that reason i think you have to fold.
Small flop bet is good, but I would usually bet bigger on the turn though with anything I'm betting here, not just the nuts.
I now fold this river. I know this is completely unscientific, but for all the times I've had hands play out like this against fishy opponents, I've never been right in my river call. As cocktails mentioned, he has to have a somewhat advanced thought process to get there with a made hand and then bluff diamonds. Most often he will check, hoping you will check back too.