[Spee]

I'm probably letting this go with thought that he is holding the 8 of spades and possibly another spade as well. There are simply too many cards he could hold that beat you.

You may not be prepared to risk another 10% of your stack here, and your opponent may know that you know this. So there is probably at least some chance that he is bluffing.

If you think he is bluffing then you have to randomize a decision to either call or fold (preferably in advance of the river card so your choice is truly random and not biased).

Regardless, I also make a mental note to play a stronger hand similarly if the situation happens to arise in the future.