Odd discharge for weeks after surgery

[Bertie2109]

My son had the ureter reimplantation surgery back in November. We've had a few follow up visits with the surgeon, but he's been unable to tell us what's going on with our son. Every 8 or 9 days, our son experiences some major abdominal discomfort. Within the next day or so, he'll begin to have very cloudy urine -- you can actually see the particles floating around in it. Over the course of the next day, he'll often have a white mucous-like discharge at the beginning of each urination. Eventually, these episodes will culminate in one to three urinations that have stringy, bloody discharges along with the urine. The blood in the urine is usually brown rather than bright red. The stringy discharges have almost a rubbery consistency to them -- it's not just congealed blood or pus. He's been on antibiotics since the surgery and hasn't had any infections. I've taken samples of the white discharge to the doctor, but my son's urine analysis have come back normal. As soon as he passes the bloody discharges, my son immediately feels better. He's only 3 so he can't really describe what's happening to him. I can't tell if the pain comes from passing these discharges through his ureter or if his bladder or incision are the causes. The doctor seems stumped. Have you heard of this happening? What are some possible causes? Thank you for any information you can provide.