Hello I appreciate your concern some times burning and stinging sensation may occur with concentrated urine symptoms disappear with intake of adequate water, there are very high possibility that this could be due to passing of highly concentrated urine through urethra another possibility is UTI, Urethral injury or prostate related disease In my opinion you can ask your doctor for urine microscopy and urine culture if it has not been done earlier He should take lots of fluids including fresh fruit juices and lemon water Maintain good hygiene ask him to wash his hands with antiseptic soap or solution every time you use wash room. Hope this helps you thanks and regards