My puppy is a 18 week old mix Australian sheppard, and he has roundworms - first sign on Sunday. I brought him to the vets for a deworming on Monday 8am, and he also got his first vaccine, so since Sunday he will not eat a thing. He is lethargic and vomits about once every two hours (few worms), and he has watery red/brown stool (few worms) - I have been MAKING him drink fruit gatoraide with a turkey baster - but he is fading away
What do I do? Are they often sick for days after?

WOW, do not force feed him anything and get on the horn to your vet immediately!! Be sure to tell him everything you just told us, especially the part about the gatoraide in a turkey baster.

Are you sure the puppy only has round worm? I hope he's OK, I am wondering if he has whip, worm , which would cause a stool the color you explained. Parvo, or Coccida maybe?. throwing up is never a good sign eighter But if needed to replace electrolytes, I use pedialyte, instead of gatorade.