Actually I would never, ever recommend whether a dog needs to lose weight based only on its weight, sight unseen, with no further detail on whether the dog is fit or fat, and especially not when a vet has said the dog seems perfectly healthy and in good weight.
This is a critical point I cannot emphasise enough -- no one should ever assume what a dog's weight should be based on a breed standard! Many dogs fall under or especially, over the standard. Even amongst many show breeders now the trend for males actually is to be ABOVE the breed standard -- many feel 18lb is too small for a solid male. Judges do not seem to have a problem with this. It is probably time for the weight aspect of the breed standard to be reconsidered, given that many judges have no issues with these larger dogs (and given that some feel there may well be strong health arguments for a somewhat larger cavalier...).
I have indeed seen a cavalier that large in perfectly good weight for his size, come into my own rescue and we have had members here with dogs around that size too.
**** If your vet thinks he is a fine weight for his size then I am sure he is fine and I would follow the vet's advice. **** You will seriously underfeed him if he is the correct weight for his size yet you feed him only as much as a dog a fourth smaller would need. You need to feed him according to what his healthy size and weight is.
As it sounds like the vet suspects other potentially serious heath issues, having hi in healthy weight and not underweight is important.
There are some oversized examples of the breed, definitely!
-- but also, if the exact background of this dog is unknown I'd suggest it more likely he is actually a first or second generation cross from a larger spaniel breed like a springer, or a welsh springer, or a cross with a Gordon setter -- which are setters with the same B&T colouring as cavaliers. If the dog did not come from a reputable show breeder, deliberate or accidental crosses are very likely. These other breeds have larger litter sizes and hence allow the unscrupulous to make more money on selling more puppies. It is easy to fake a pedigree/registration (so any breed could be in there despite it saying he is a cavalier) unless the dog has been DNA identified.
Maybe post a picture or two?
The only thing that matters is that you have a dog you love and are clearly giving love and care to a rescue dog that badly needs it. The size or even whether he's a purebred or not surely doesn't matter. The size of your heart in giving him love and care does!
