Hello, Thanks for posting your query. Hydronephrosis means swelling/distension of kidney, due to retention of urine or inflammation. It can be due to lower urinary tract infection which may ascend to kidneys causing frequency of urine and distension of kidney/ureter. stone in ureter can also cause distension of ureter and kidney but would have been detected on sonography. As your child has had surgery for congenital abnormality, he is at higher risk of UTI and obstruction by post-surgical strictures. Give him cranberry juice which may cure and prevent UTIs. Get his labs done, urinalysis, urine culture and dye scan. UTI can be treated effectively with antibiotics after urine culture results. Hope it helps. Take Care!