Does he look right at you, with that "haha, got you" attitude? (We all know which look I mean, LoL)
Or is there any chance that this is still due to a combination exposure to cool air and being reminded about the body function?
You know what, either way, I would start doing the changes in the bathroom, as the others suggested. He doesn't lie down like the babies do, he is a big boy so he goes in the bathroom. Whip the diaper/pullup off and he is on the potty. He can do his peeing in there.