Just when you think your puppy is nicely potty-trained, he pooped on the kitchen rug..

SIT is ok (mostly), COME HERE is intermittent (50/50), FETCH is not a concept he grasps yet.
How DO you train a 2 months old puppy? Too young to bring to Petsmart (vet said not fully immunized -- not allowed to mingle with other dogs). Any potty and other training tips? Please? Tha!