5 year old passing blood clots

My 5 year old son has passed massive (6-10) blood clots on several occations during bowel movements.  I took him to the doctor after the first time and the nurse practitioner told me not to worry and that it was probly just a fissure.  He was constipated , so I have added fiber to his diet and for the past few months he has been having recular bowel movements.  I have lost medical insurance on him because my husband lost his job in Feb.  I am so afraid of what this maybe that I think I am in denial to an extent as I keep putting off getting him seen.  The blood clots are dark red and big,  the toilet also turns a bright red after he goes, and there has been mucus mixed in with the blood.  whe this happens the blood literally runs down the back of his legs all the way to his ankles.  Does anyone have any similar experiences with this?? I have searched the net, but there does not seem to be too much on this subject regarding children.  I plan to make him an appointment first part of next week with or without insurance.