My 4 Year Old Son Doesn't Want To Urinate. It

My 4 year old son doesn't want to urinate. It usually take a lot of coaxing. He probably urinates 2 times a day. Often times he cries out in pain while urinating, which is surely a good reason not wanting to urinate, if it's painful for him to do so. It varies from mild discomfort to true bad pain. He states that his "hip is hurting". Sometimes one side, sometimes both. It's actually right above his hip bone on the side of his abdomen.
He had enlarged kidneys while he was unborn and as an infant, but it seemed to resolve itself. Monitored via ultrasounds for the first year and half. He has been seen by a urologist the last time because he just wouldn't go. He had to be hooked up to a catheter. He was going 24 hours without urinating and we suspected a blockage, but he CAN go. There's no block. What other conditions could it be? What would be causing pain?