[Doc]

The key for me is that he can't have the nuts - because you have the Ace of diamonds.

Your bet of about 3/4 of the pot is perfectly consistent with having the nut flush. If you re-raise him all-in he has to give you credit for the hand. His raise is offering you more than 4-1 to draw a card - with position! It makes no sense for him, IMO, unless either a) he has the nuts and wants you to play with a lesser flush or
b) he has some other hand such as trips, a weak flush (e.g. 76s) or a draw to a lesser flush

I think he is just looking for information with a non-nut hand. He might have trips, but he'll have to let it go in the face of an all-in here where you have showed nothing but strength. There are just too many ways he is beaten. KQs is about the best case you can make for him, and even that says weak player to me - wanting to play an average drawing hand out of position against two players who have demonstrated strength.

All-in it is, and even if he does win - you'll get the money back from a guy like this.