I am all for telling the truth.
Now, you met this dude, and hooked up. You may see him again or not. This is not purely your game. It takes two to tango. I doubt that I would want to sit at home, and depend on his phone call, txt, meet up, whatever. So, I'd go out and meet someone else. Would I be telling this 'new' dude that I had met someone else, who may or may not be showing up anytime soon? Nope.
The default is that a dude is a single, unless he says otherwise. What a single dude does is his business, and he owes it to no one to relate the stories of his latest conquests. Actually, doing so, would seem somewhat tasteless in my books.
Once you start meeting up on a regular basis, and either guy feels that he is about to enter the serious dating stage, you want to talk the details. Not before not later.