My son whos three has yellow mucusy stringy bm's why? Most of the time but sometimes very dark brown. Hes not strainin to go. Its formed.

Food allergy. Very often gut inflammation from undiagnosed food allergies leads to gut inflammation which presents this way. Start a daily food diary to start tracking foods and then go to a4m.Com and use directory function to find a local functional med doc to help you evaluate this and maximize your son's health. Start him on a high dose multi strain probiotic from the health food store in the meantime.