First of all, I'm so sorry you're son is going through this, even though he's more than likely unaware of what's going on and won't know that it even happened in the years to come (silver lining that it's happening to him prior to his memory banks turning on!).
I suggest the following reading: http://www.cincinnatichildrens.org/health/info/urinary/diagnose/ureteropelvic-obstruction.htm
If it's a ureteral blockage or a narrowing of his ureter causing hydronephrosis, although a delicate surgery, it's completely operable & correctable. The good news is that it's only on one side, so only one kidney is obstructed/affected, and the other one is performing well (I assume so, as you didn't mention otherwise in your post/question).
I personally have one bum kidney, but my other side is 110% (perfect function). The human body is an amazing thing, with built-in redundancy in the case of the urinary system, so as long as his other kidney remains intact, he should recover well.
I'd like to hear back on the diagnosis, once the doctors figure it out, so please post again soon.
Cheers.