Hello, I'm Dr Gary. I've been practicing veterinary medicine since 2007. I look forward to helping with your questions/ concerns.
The change in your dog is concerning. I don't think it's all related to diet, but I would change him back to an adult food. It doesn't have to be Dog Chow, but I would go with something made for adult small breed dogs. I would go with Iams or Science Diet or Eukanuba small breed adult. You can continue to feed the puppy his Puppy Chow, but you don't want to feed an adult dog puppy chow.
The lethargy and weight loss is concerning. With those signs, I would get bloodwork run. I would check him for any signs of kidney disease, liver disease, diabetes
, Addison's (low cortisol secretion) or Hypothyroidism
(low thyroid function). These are all possible causes.
If something shows up on the bloodwork, then management can be discussed at that time. There are many things that can cause lethargy in dogs, so it's time to rule some of the common things in or out.
I hope this helps, let me know if you have any other questions.