Hi there
Firstly parvo is much more likely in very young or very old dogs- has your dog had any vaccinations at all? If he has it's very unlikely.
There are actually quite a few causes of HGE (haemorhagic gastroenteritis) and parvo is only one of them. There are many bacterial infections that can cause these symptoms as well as inflammatory bowel disease.
Dogs with parvo also vomit profusely, so again if he's not vomiting, then it's less likely parvo and more likely another cause of HGE.
The main treatment in any of these cases, whatever the cause is to keep the dog hydrated- so be sure he's taking fluids- plenty of them- you can add a pinch of salt, pinch of bicarb and a tablespoon of sugar to a pint of water to make a good rehydration supplement, or buy human Diarolyte.
However, cover with antibiotics is also often vital in these cases, as even if the initial cause isn't bacterial, you often get a secondary bacterial infection- so i would advise seeing a vet to get this side of things covered.
The main thing to concentrate on at home though is fluids- small amounts and often to prevent dehydration- which is the killer in most cases of parvo.
Regards Andrea
Dog Veterinarian
bvm&s degree, 15 years in companion animal practise