There's a progressively bigger bulge on his chest in the upper left* area which then explodes in a spray of blood.
It's about as clear as possible short of an X-Ray or him shouting "No, my heart!!" that it's his heart exploding.
*: I have, for some reason, found a clip that inverted the scene so that it instead appears to be on the right side. Either way, it's far too high and to the side to be the weak spot which is in the center of his body. And why would the weak spot bulge out before exploding?