Aww, I'm so sorry.
There are great reports on Azodyl but I'm not sure it would work in this case.
That BUN isn't terribly high and it he wasn't allowed water before surgery, it can be up from dehydration. Does he have an infection?
Who did the ultrasound? An internist or your regular vet?
Were they just small or what?
Patti (YT member) may have some ideas for you.