Diagnosis Of Child With Painful Urination Despite Negative Urine Infection Test?

Question: My 4-year old complains that it hurts when he pees. This started this on Saturday and we took him to the doctor on Monday and his urine test for infection came back negative. Doctor suggested he was constipated and he told us to give him Mylarax. My son started on that Monday and he has been pooping twice on Monday (only a little bit but didn't seen to be a hard stool), Tuesday (loose stool once) and today (twice also loose). However he still complains that it hurts. No fever, no other symptoms, eating normal, active. Urine is normal in color and in volume. He goes more often than normal. Also, the pain seems to appear no when he starts peeing but at the end where he suddenly stops and he says that it hurts. No injuries in that area. What other diagnostic can be?, should be wait longer until Miralax completely work?. Thanks

Hi, Doctors tested his urine to see if they can find some blood that would indicate that maybe he has a stone and results were normal. What do you suggest? Thanks