Darkness on neck , poor weight gain. 11.

My 2 year old son had congenital unilateral hydronephrosis. And since 1 year he had darkness on neck, poor weight gain , drinks plenty of water and pee a lot . Night time also drinks so much water pee a lot . Is this any symptom of prediabetes, my son is so much active,  constipated, 2 months back he went a pyeloplasty surgery, his recovery is so fine , since birth we give formula milk and he used to eat lot biscuits and chocolates for 5months after he is constipated we completely avoid giving him biscuits and chocolates and any type of sugar items .his birth weight is 2.75 , at 12 months his weight 9kg , and after that he is 10 kgs only not gaining weight, now his weight 10.500grm , after pyeloplasty surgery his weight is 9.7 kgs after 3 months now 10 .500 grms and doing well . Shall I concern about his poor weight gain . And his hemoglobin is also low ,in Feb 26 it is 7 and may 26 it is 11.