What Causes Vomiting And Painful Bowel Movements In A Child?

My grandson has just turned 6 years old. He goes 2 or 3 days without a bowel movement. When he does need to go, you can tell because he will go off by himself, sometimes sit on the floor cross legged. If you ask him if he needs to go to the bathroom, he hesitates. I am sure it s because each bowel movement is painful. I would say, about half of his bowel movements are large in size, like over 10 inches long. Like he is so bound up. Recently, he has been vomitting, so not real sure if this is from his bowels or because school started and he has been exposed to all the new school germs and illnesses.