Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you today. I do apologize that your question was not answered before. Different experts come online at various times; I just came online, read about your situation, and wanted to help.
Again I do apologize that my colleagues could not aid you sooner. If you would still like assistance, can you tell me:
When Hugo passes urine is it large or small volumes? Any change in odor or does it appear watery?
What tests did his vet do?
Did he use the box again while on treatment?
Does he use the box for stool?
Any changes to the box itself (ie new litter brand, scent, location, etc)?
When did you change the actual box (ie after signs started)?