Is he having diarrhea? How is he feeling? Does he just lay around?
Did they test for Parvo? How many vaccines has he had?
If they did test for Parvo and it's been a couple days, I would re-test. Early parvo can be negative. I'd also do a CBC - a low white blood cell count is usually very indicitative of Parvo.
Regardless, I'd want that puppy hospitalized if he were mine. At 8 weeks with continuous vomiting, he can't survive long. He'll need IV fluids and injectible medications.
Coccidia can cause vomiting and diarrhea, but my first suspicion in an 8 week old puppy with symptoms this severe would be Canine Parvo Virus. I really would recommend taking him back in and doing it now if you're able.
I hope he does well for you. Hang in there.
