What Causes Stomach Pain And Constipation In Children?

Hi, I have a three year old son, we were advised to go to a&e two days ago which we did, he has had on and off stomach pain causing obvious discomfort, he has no fever, has vomited only once but is not complaining of feeling sick, just stomach ache. His urine was tested, no infection and he is not dehydrated. He is still having this pain who his horrible for him to have to endure. He has not passed stool for three and half days now. What do you suggest? Thank you