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if your son is otherwise active and looks healthy, do not worry. There does not seem to be a major illness. If his appetite is good, and there is no weight loss
, relax. You need to make him sit in toilet, irrespective of whether he passes motion or not, because bad toilet training is the major cause behind this. To begin with you may have to carefully increase his dose of laxative gradually and increase his fiber intake like pulses, fruits, green leafy vegetable, and plenty of fluids. Once toilet training is accomplished, laxatives may be weaned off.
Have patience, take care