Well, FLUSH IT, if he didn't have the courtesy too, the you do it! Of course if he didn't, then dump his butt, that means he has no respect for you, but seriously, what does it matter?
We all have to go and it's a natural body function, so what does a guy doing a smelly poo in your toilet have to do with anything? What do you do hold it in until you burst?
I know some believe that etiquette should say a guy should "not" use the restroom at his "dates" house if he has to do that, however, I see no ill will about it, I mean we all go and no it's not a pleasant thing to go into a restroom after someone just took a good steamer and smell that, but realize, WE ALL DO IT, anyone who says they don't are liars.
Give the guy a break and next time supply a little air freshener in your bathroom and if he doesn't think to spray it, then you do it.