I could not tell WHY he started having an awful breath after the cleanse, but I can tell you that bad breath is due to bacteria proliferation. Such proliferation often occurs in presence of toxins (which we are trying to expell with cleanses).
Go to your local health store and purchase coconut oil. Then Have him gargle for 15 minutes with a spoonful of the oil twice a day. Make sure he doesnot swallow any of it but throws it a ll out and rinces. This is called oil pulling. It is a protocol to help get rid of toxin, we usually do it with any vegetable oil. He could use any other oil (much cheaper), the advantage of using coconut oil is to kill the culprit (while expelling the toxins) and thus help with the rancid breath.