Hi there, I am Sophie a registered veterinarian and I'd like to try to help you.
I'm sorry to hear that Taz is loosing weight. Weight loss is a common symptom in cats over 8 years for a range of different diseases. The most common conditions causing weight loss in these guys would be diabetes, kidney disease and hyperthroidism. There are of course many more, but these are by far the most common. Symptoms are commonly accompanied by increased appetite and drinking. Parasites (worms etc) can also cause weight loss. These diseases can also cause high blood pressure and blindness resulting in incoordination.
A simple urine test done by your vet can be a great screen for diabetes and kidney problems. They can test for additional sugars in the urine which may indicate diabetes and the concentration and levels of protien in the urine to look at kidneyh function. Well functioning kidneys produce concentated urine without protiens present. A blood test can be done to check the level of thyoxine in the blood and can determine if your cat is hyperthroid.
Cancers are obviously something to be considered in an older cat as well as viruses your cat may have contracted. No advice will be as comprehensive as a full physical examination by a veterinarian.
A good physical examination from your vet should be able to come up with a more comprehensive list of differentials and your vet should be able to do the urine sample the same time.
I hope this information has been of some help to you. Please let me know if there is anything else I can clarify or if you need more information - just hit reply. I wish you the best of luck, do let me know how you get on. Many thanks, Sophie