Mark - Integra and Kattovit from Zooplus are worth a go, I think it was Integra I tried with Rosie, she would eat that whereas she wouldnt eat the 3 RC and 2 Hills foods, and the one I bought came in packs of 6 trays, so not a large amount to waste. I would also give the vet another call back and ask for the ranges, that way you know exactly how much higher they are - I know the high end of normal is similar at my vets to Jane for in house tests, but I think the lab ones are different. If that is the case, the Urea isn't too bad, Rosies was higher than that at diagnosis, hers was 252 and Creatnine was 20.9, on the CRF site, 250 is Stage 3.
As Carol says, it does depend on if he was starved, and also if they were done before the vet gave him a steriod jab, as they can affect the kidney readings.