Even though the other puppy recovered okay, I would not take the risk with this puppy. At 2 days, you are running the risk of dehydration, losing muscle
mass and hypoglycemia
(low blood sugar) complications.
Possible causes could include viral disease (parvo
) even if he has been vaccinated once or twice, intestinal parasites is very high on my list (not just worm issues, but the very common microscopic bugs like giardia or coccidia), or some dietary problem.
So, definitely, I would get in to a vet today for supportive care.
To help the stomach
and intestinal tract you could give a 1/4 tsp of some pepto bismol. To help prevent low blood sugar and when lethargic, you can give a 1/4 tsp also of some pancake syrup or karo syrup. If weak due to low blood sugar, the pancake syrup should give a boost of energy within a short time.
But, right now, he is not even tolerating water, so that is your red flag, to get a vet involved before he deteriorates.
Very sorry that he is not feeling well. Good Luck!