Goodness I would indeed get to a new vet immediately. If he has pee that truly dries black then this sounds as if he has blood in his urine and could be an emergency. On the other hand maybe it just dries leaving a slightly darker mark depending on where he pees and it seems as if it is black? But really, what you are describing sounds like potentially a very serious issue and I would not even consider waiting but would get professional advice -- I'd also be looking for a new vet and an appointment for first thing tomorrow or trying to get in today.
Possible causes of blood in urine can range from urinary tract infections to crystals in the urine to bladder or kidney problems and all can turn very serious in a young pup or signal a very serious underlying condition, so do get him in as quickly as you can (eg within 24 hours, or to an emergency vet). If the urine is actually drying black it sounds like a very large volume of blood though!
If he has scratched hair off his hindquarters the most obvious causes are a flea allergy, rabbit mites (has a skin scrape been done) or mange. A lesser possibility would be an allergy of some sort and the most unlikely issue in a young pup, but one every cavalier owner needs to be aware of because of the high incidence in the breed, is syringomyelia -- there's more info at
www.smcavalier.com but also info in the Health Library at this site.
The most immediate concern however is blood in the urine and from your decription perhaps a very heavy amount. I would personally get my dog to an emergency vet immediately to figure out what is going on.
Also if you mean you are feeding only just chicken and rice (eg preparing this at home), this would be an inadequate diet for a dog and especially a growing puppy. He needs a complete food during this very important growing period in his life --he may have a nutritional deficiency that will affect his immune system. A vet can advise you on this as well. I'd get a puppy on to a complete balanced dry food or dry with some added wet food, meat etc.
Good luck and please let us know what you hear back.
