His limping in preflop likely indicates a small pair unless he had been limping in a lot where his hand range is much wider. Axh is possible, so is a hand like T9h or something similar, but I really don't think the nut flush would reraise on this flop, at least not such a big raise. If you call and another heart comes you could be in trouble and you definitely won't know where you're at, and if the board pairs you could be drawing dead and not know it. I would be looking to get a lot of money into this pot while I'm ahead.

I'm curious to hear the results though.