jack russell pup 12 weeks old not eating

Question
My jack russell just turned 12 weeks old was doing fine we dewormed him with product from feed store he had tape worm the next day we took him to the vet he had first parvo shot from there it has been down hill he was eating drinking pottying but last few days he just lays around wont eat or drink he is the runt so was small to begin with dont even weigh a pound and now loosing weight fast have vet apt for fri but need to know what i can do till then or if any idea if this was from the meds we or vet gave him please help scared for loosing him

Answer
Call your vet immediately and get an appointment TODAY. Tell them you're not waiting and if they can't take you, go to a different vet.
He should have been dewormed at 7 weeks of age with appropriate medicine from a veterinarian, and then again at 12 weeks with the same medicine. The parvo shot should have been done earlier too.
If he doesn't drink, especially at his age and size he runs a high risk of dehydration and death.
I'm serious you need to get him to a veterinarian NOW.
I hope everything is OK.  DON'T WAIT UNTIL FRIDAY.
Good luck!!!