I'd just stop, quietly and discretely let the gentleman know that his jacket was on upside down and offer any assistance if he'd like to re-arrange it. I believe there are ways to offer anyone assistance without coming across like you are doing them a favor or that they can't manage or must take your help.

Maybe the real favor isn't from you to the man in this instance. Maybe the gift is that you cared enough even to notice and offer assistance (even if you end up rebuffed).