alex,
The way I see it, in your example, it would depend on his position in the betting round. If he were first to act and went all-in, SB position post flop, he would showdown first. If he were in a different position. e.g. UTG+1 and the action were checked on a later round after he went all-in, then the first to show would be the first player that checked.This is getting confusing to explain but, I believe I'm getting it right. Consider a player going all-in but calling the exact amount that was bet in front of him...hmm.
The rule I've been looking for, (and unable to locate), called for the all-in to show first no matter where he was in relation to the button, as long as there were no side pot. I have been unable to find it, so I think the standard mentioned by Chet is your best choice. Last to initiate a bet on the last betting round, or the first player to check on the final betting round...that's the way we do it.
For those that read this earlier, I made a couple changes to my original post. I think it makes more sense after my correction.