What Causes Constipation In A Child?

sir my son aged 2 and a half year, has a sever constipation since 2 months,he is eating rusk(papay), cake, biskits and polybag juice slice of pepsicolla.He is also using 3 feeders of cow milk and some amount of wheat bread sometime a day, he passes stool after 5 days with great pain. most of the time the stool was scooped out by me with my finger.a little spoting of blood also came with the stool.lase time a noticeable amount of blood trickled down fron his anus.he had used polybion syrup and setran syrup but no cure. now dr is giving him lylic, polybion and ibret syrups and zyocain gel for anal insertion. still 5 days have passed and he didnot pass the stool.(gel was not given up till now)