Hi,
How much water does he drink in an average day ? Excessive thirst can be a warning sign for serious conditions. I don't want to worry you but take a look at below ...
If you have problems funding any further tests it may be worth contacting the Blue Cross. I don't know what the criteria has to be to qualify but they do help people who cannot afford private veterinary care.
"It is generally not thought to be a problem if a cat drinks less than 100 ml per kilogram body weight per 24 hours. If your cat drinks more than this amount, there is nearly certainly something wrong! It has to be remembered that the water intake of a cat is dependent on many factors, such as the weather, the type of food (dry or tinned), the activity level of the cat or the physical status of the cat (pregnant, lactating etc.).
Increased thirst is nearly always related to increased urination and some of the conditions mentioned below are primarily conditions causing increased urination and secondarily increased drinking.
One of the more common reasons for cats to drink excessively is kidney disease, especially in the older cat. Liver disease is another common reason. Diabetes Mellitus is also possible in cats. Furthermore, there are several hormonal conditions which can cause excessive thirst, such as Cushing?s Disease, Diabetes Insipidus, Hyperthyroidism, Hyperparathyroidism etc. There are some more, less common, conditions as well, causing excessive thirst.
In most cases, your vet will have to do several tests, mostly blood tests, to find out what the reason for the excessive thirst is in your cat. It is always advisable to visit your vet if your cat drinks excessively."
Expensive vet bills are not much fun and it's a shame that you cannot get him insured as it was an existing condition. But it does sound to me (and I'm an ex Vet Nurse, many moons ago I may add) that there is something underlying. It's a case of a good vet piecing together all the problems rather then maybe looking at each individual problem.
He may scavenge as force of habit having lived with so many other cats and having to fight for food ? Just a thought.
Anyway, keep me posted. I'll be interested to see he gets on. Do contact the Blue Cross though, you've nothing to lose. Good Luck.