My small puppy has diarrhea and is lethargic. When to see a vet?

It always concerns me when young puppies get lethargic. Puppies are more susceptible to the effects of dehydration and hypoglycemia (low blood sugar) than adult dogs, so diarrhea is more serious for Wallace than it would be for an adult dog. If he's back to acting normally, eating, drinking, and doesn't have further diarrhea or vomiting, it's perfectly fine to continue monitoring him at home. However, if he remains lethargic, has more mucousy diarrhea, begins to vomit, or refuses to eat or drink normal amounts, then you should have him examined by his vet. Your vet will need to do a physical exam. Depending on their suspicions based on their exam findings, they may recommend some diagnostic testing or may opt to start him on some medications. Possible causes of this type of issue include stomach upset from a virus, intestinal parasites, among many other issues. If your vet finds that Wallace is dehydrated, they'll also be able to give him some fluids. I hope that he feels better soon!