Using this product on puppy five weeks old

Our five week old puppy vomited up round worms. We gave him a dose of Pyrantel Pamoate. The dose prescribe was 1 teaspoon per 5 lbs of body weight. The puppy weighes about three pounds so we gave him only a half of a teaspoon.

After he licked up the medicine we fed him. Up to this point he had been having loose stools. However even after being fed another time he can not go to the bathroom. When we put him down on the ground he acted like he wanted to go, but couldn't.
Also he is lying around very " puny " acting. Is this normal or could he develope a blockage that quickly ? If or when he passes the worms, when should he be given another dose?