2 year old cat suddenly has difficulty walking and is very tired.?

The cat was very active, inquisitive and a big jumper. In a matter of 2 weeks, he started to get clumsy and then could not get up and walk. He then spent most of the day sleeping. We took him to the vet who did X-rays, blood and urine tests which did not find anything. He still has an appetite and drinks water (although, not as much as usual). The vet is now recommending a whole series of expensive tests (we are in it for $500 already) and indicated that they may not show anything up. Does anyone have any suggestions or have a similar experience?