There are a few things the poo could be, one of the most common is hexamita, which is an intestinal parasite. If he is eating, that's a very good sign, as it will be easier to treat him and it means it's not too far gone. The recommended treatment is to feed pellets soaked in Epsom salt solution. The salts will clear his system out. If that doesn't work, metronidazole is more effective but more harsh to the fish as well.