Hello?
I am not a reproductive specialist. But I think your vet can ask their lab services rep about progesterone checks or go get an ultrasound or refer to a surgeon for help. These cases can be frustrating but it is really important to make sure a testicle was not left behind in the abdomen.
Hello
Call alll of the local vets and rescues/shelters to see if they can recommend someone. Also I have had some success with medication and cold laser therapy. An X-ray yo look for urinary stones in the bladder is also an option. The cats with urinary stones often need a prescription diet. Weight loss and encouraging water intake, or switching to a high quality canned cat food and adding water can also help
Lastly I would like to know which vet clinic you went to by adding your story and the cost of care to the story line section available here.
Let me know what happens