Okie dokie - yes, I COMPLETELY buy into the idea that the fries are the culprit on the constipation side of things. Does that to my oldest, too. (But not my youngest - ah well, eveyone's different and my yougest has a gut like a stellar black hole. Nothing can escape...) Here's the good news, that is info you can use later on! i.e. the next time his poop is a little on the loose side, it's time for some therapeutic Micky D's! ;-)
Can't say I know much about a "tummy cleanse" for a little guy, but I used a little flax oil quite sucessfully to soften up poops with my second son. (ironically, oranges of any sort usually work like magic with the French fry sensitive dude.)
One other thing that helps me (and how I found out about the correlation of fries and the oranges) was to write down everything that they eat and when they poop. It only takes a couple of seconds and I just put it on my schedule so if they have problems, I can look back at what they've eaten to see if there's a problem.
Hope this helps. (Sorry you may have seen all this with one of your other posters. I didn't check them before I wrote.)