That almost sounds like it could be an umbilical infection. Of course, an infection would spread very quickly in an infant. The dr was negligent for sending him home, IMO.
If his bladder was full and he couldn't pee....wouldn't you think this would be a bigger risk to have a child die so quickly? Urine backing up into the ureters would shut down the kidneys..and would lead to a multi-organ failure. My guess is the Doc may have botched the circumcision and damaged the baby's urethra so he couldn't pee. But... all is speculation without seeing an autopsy report. How tragic for these parents......