Hello Paul. My name is***** and I'll do my best to help you today.
I'm sorry to hear about these symptoms in Tiddles; I know how distressing this must be for you.
I'm also afraid that my answer is most likely not going to be a particularly positive one since this combination of symptoms in a cat this age usually doesn't bode well for the patient.
The possible explanations include:
1. Systemic disease such as diabetes with secondary organ enlargement of spleen or liver. Diabetes can be treated with insulin and diet but is often quite difficult to control especially when it develops in older cats.
Pri***** *****ver disease can also cause the symptoms that you're seeing; blood work usually indicates elevated liver enzymes, an ultrasound may be useful but usually a biopsy is suggested.
Kidney disease can cause all of his symptoms except for an enlarged abdomen. But sometimes when weight loss occurs, the abdomen appears enlarged when in effect it's not.
2. He could have an abdominal mass which is going to be cancer, I am sad to say, until proven otherwise. Such a problem could explain his lethargy, weight loss, abdominal enlargement, and decreased appetite but increased urination isn't typical in these cases unless there has been spread to his kidneys.
As you might expect, the prognosis would be poor if this is the case.
Treatment is often aimed at improving quality of life such as appetite stimulants, anti-nausea medication (if the patient is vomiting), etc.
3. He could have fluid buildup which could be secondary to heart disease (ascites) or cancer. We do have fairly effective heart medications which can be given if cardiac disease is diagnosed, usually with an ultrasound.
4. He could have FIP which is a virus and is a very bad disease. The abdominal distension would be secondary to fluid. This can sometimes be a challenging diagnosis to make although evaluation of the fluid can be helpful.
It's possible that a good physical exam will pinpoint the problem or additional tests which I mentioned above may be needed or suggested.
It may help to know that he's not likely to be in pain per se but fairly uncomfortable as you might expect.
I realize that my answer may not be what you want to hear but I would be doing a disservice to both you and Tiddles if I were less than truthful and honest in my response to you. I hope you understand.
I also hope that this is helpful in providing the possible explanations for his symptoms.