[Cumulonimbus]

I think this is a bad fold on his part; most players that make moves like you did either have a high pair that they want to defend against a spade draw, or a high spade themselves that they'd like to semi-bluff with(the Ace in your case.) I would have assumed my set to be the best hand, and if I was down to a flush, I still have 7 outs to the turn and 3 more outs to the river.