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Your son's bee­n having mucus for three years - that's not normal. Mucus can me­an allergies, infections, irritants. He­ might cough, clear his throat, swallow a lot. Drink water, use a humidifie­r, stay away from smoke. 

It seems like the kid is constipated. This is when they find it hard to pass stool. It may be due to the type of formula they are taking or insufficient intake of fluids. You can give them some water in between feeds or massage their stomach gently to relieve the discomfort. If it persists, it would be best for you to take them to a doctor who can check for any underlying problems.