Please cut down on his milk (not > 400-500 ml/d) and increase cereals and fibre diet. You must give him khichdi, dalia, mashed vegetables, fruits like papaya, guava, sooji / besan halwa, etc. Honey is a good alternative for sugar. You can put him on Lactulose syrup to soften his stool. You need to visit your paediatrician who will provide you with these details.