As a top who ran into this situation -- it's your duty to tell him.
To those of us who don't fetishize HIV.... it's still considered to be a BIG FUCKING DEAL. So much so, that if I hit it off on a date with somebody, and something's about to go down without a condom... I would expect the bottom to tell me if there was a risk I should know about. He assumed that everything was cool because you didn't stop the proceedings. I don't care that he did it anyway, because by not disclosing, YOU put him at risk.
When I was in this situation, the guy and I were both drunk... there was a miscommunication about statuses. And that was that. So, being sober -- yes, I think you're scum.
Sidenote, if I was in his situation and found out later -- I would [...]
[edited by rawTOP - be nice to each other guys...]