Review of history doesn’t reflect any correlation between the “kidney pain” and intake of meal. Imaging tests of kidneys and the ureter does not reveal any evidence of obstruction in the urinary tract system. Your son does have bilateral extra renal pelvis, (a part of urinary draining system between the kidneys and the ureter) which is a normal anatomic variant since birth. Unless the imaging studies reveal the swelling/obstruction of the tract system at the time of the pain in the referred area on the back, it is unlikely to have any relation.