oh wow...I'm so sorry he's this sick...Petland is famous for selling sick puppies and I'm sorry you're going thru this
...did they run tests on him for Liver shunt ?? Coccidia & other Parasites ?
I'm no expert but considering his young age - you should call the vet asap - The weekend is coming - do you have an emergency vet close by ??? time is pretty important here if he's gone this long not eating.
Poor little guy - this seems to be happening so often lately ...there are some other threads about puppies with similar symptoms but I think you need to make a call and see if someone will see him right away