I agree that the smell and color change would be the part that's really concerning, if nothing in his diet has changed. Has something in your diet changed that he may be reacting to possibly? My son doesnt have sensitivity to anything in particular that I eat (which is awesome because I LOVE spicy foods) but some babies apparently can be sensitive to certain foods their mama eats. So if you've eaten anything new lately, or something you hadnt eaten in a while, that could possibly be part of it.
As for how often babies poop, once they get past the newborn stage, it doesnt have to be so cut and dry. My BF 6 month old went from more than one poop per day to just one big one, and now he sometimes goes a day or two without pooping. His poops are healthy and smell fine (except when he's eaten baby food, then watch out for the smell!) and I know he's fine. Babies are humans, and lots of people dont poop every day, which is fine according to Dr. Oz. Of course, more than a couple days and I'd start to worry about constipation.
I hope the doc is able to help figure out the specific issue!