Something just doesn't seem right with your pup.
A five-month-old shouldn't need Nutrical to stimulate the appetite or stop lethargy.
The average water intake is about an ounce per pound per day.
So he should drink about 5 ounces or between 1/2 and 3/4 cup per day.
Did you say he drinks about 1.5 cups per day?
That is a symptoms of liver shunt as is lethargy and inappetence.
Your pup really does need to see a vet as soon as possible.
Have you tried canned food to entice his appetite?
It is good that he is eating now but 1 cup at 1 sitting for a small puppy is way too much in his stomach at once.