my son is 21months old and his bowel is normally once a day. but occasionally when we travel can stretch to 2days once and the texture is hard and dark in color. i think when we travel we might not gave him enough water as when at home.
you can try to give him more veggies and if he eats bread choose wholemeal one. for fruits you can try banana and papaya which are high in fibers. also avoid heaty food like chocolates, mangoes, longan..
give lots of fluids too, beside water can give fruit juices. also can make his milk slightly diluted.
hope that helps