How much did he go all in for? Does he have you covered? What does "somewhat" aggressive mean?

It's not a bad fold, although I have a feeling you will be barked at for not taking a chance at what seems like a push. Here is the thing - with three diamonds on the board after a cold call on the flop, the logic of his play seems to indicate that he is chasing the flush. Either that, or he slow played a set AND he thinks you are aggressive enough to bet at this thing with less than the flush AND he thinks you might have a fourth diamond and wants to push you out. Also, he thinks you don't know this. But, with the second statement, I think there are too many assumptions and you don't know your player well enough.

I would advocate a smaller bets on the flop and turn. That way, if you really want to bet less than the pot, you can make it 50 on the turn and then get out. OR, you can bet the full pot at 75 and not appear weak, giving your opponent something to really think about if he is trying for a big time push representing the flush. By making a bet of "75ish" on the turn, after betting the pot on the flop, you look weak and give your opponent and opportunity to push with position on you. And then, the action is on you to make the really hard decision.

So, I suppose what I'm saying is that your bet on the turn was the biggest mistake here.

If you insist on betting fractions of the pot, I would question, why not a bet of 2/3rds the pot both times, so you can see where you are and save $$? If you are planning to fold to a push anyway, there is no need to be more each time. Given your game plan, you could have saved 16X BB here with some better pot management.

Overall, I probably fold too, unless you had some real hard read on him.