Ok. At this point, it is obvious that something isn't well with your little one's intestinal tract. Given his age, these are the things that I'd be concerned with.
1) Giardia - a protozoal infection that causes vomiting and diarrhea. Here's a link on it.
2) Intestinal foreign body - something stuck in the intestines.
3) Severe parasitism
4) Parvo virus - hopefully less likely since you had him through his puppy vaccines with your vet.
Honestly, at this point, I would really recommend getting into your vet for them to run some tests to exactly pinpoint the cause. The metronidazole isn't helping at this point and further therapy is needed. Until you get it - which should be as soon as possible - I'd fast him for 12 hours. No treats, no food, nothing. You can offer a small amount of water after the 12 hours, but don't allow him to drink too much at one time (placing ice cubes in her bowl so that he can't gulp water will help).
After having access to the water for 1 hour, I'd offer a bland diet of boiled rice and boiled hamburger (3 parts rice to 1 part burger) in VERY small amounts (perhaps 1/4 c every hour or so) for the next 24 hours. Once you start feeding the rice mixture, monitor for recurring vomiting. If the vomiting doesn't start again, you can begin slowly mix in his regular food over the next few days until you're back to just it.
Again, this may not be enough and that visit will be important to helping him get better.