
Oh dear poor little man. Didn't the breeder give you a supply of the food they were feeding him on ?
!3 week old Cavaliers should still be on 4 meals a day, breakfast, lunch, tea & supper
Changing his food is not a good idea & I always give puppies boiled cooled water for the first few weeks as some thing as simple as a change in water can cause problems
Do you give him milk ? Dogs don't need milk once they are weaned & milk can keep the diarrhoea going
Personally I would feed him small amounts of bland food steamed white fish or chicken breast & rice cooked in the fish/chicken water until it goes sticky until the stools are back to normal & then very gradually wean him on to a quality complete puppy food. Once the stools are normal you can give small amounts of natural live bio yoghurt to put the good flora back into his intestines