Suggest Treatment For Recurring Bleeding With Bowel Movement

My son is 3 years old and has been bleeding when he has a bowel movement. His stool does not look hard or dark and he doesn t push or feel pain when he goes but he does have bright red blood dripping from his bottom. He eats at least 5-7 servings of fruits and vegetables per day and drinks water all day. I have cut his milk back to 8 ounces at night. This started about 3 months ago and I called the doctor immediately, it stopped for a while and then started again, he had several blood test and a stool culture, the doctor said everything looked normal even thought one of the test IMMUNOGLOBULIN showed a value of 155mg when the standard is 21-128mg, his ESR value rate was 18mm and the standard showed less than 15mm. My son also had an xray which showed he had stool backed up in his colon. The doctor advice I give him miralax 3 times per day, the bleeding stopped but 3 days ago it started again. I am not sure what to do or why this is happening. What would you recommend are the next steps to figure out why he is bleeding.