UTI in male children is rare, and always requires ruling out any anatomical abnormalities, as rightly done by your treating doctor. Having found the swelling in kidney, it is essential to find the cause for the same and determine the damage done if any, so that such further episodes may be prevented. The advised tests of MCU and DMSA scan are the efforts in this direction. Tight foreskin (phimosis) may sometimes be the cause of UTI in children. In a child with phimosis and recurrent infections, it may be advisable to have circumcision to prevent repeated infections, if everything else has been found normal on examination and investigations.