Hello, I am very sorry to hear that Buster is lethargic and isn't feeling himself.
Is he running a fever (more than 103F rectally)?
Trembling can be from anxiety, pain, nausea, fever, or even electrolyte or mineral imbalances in the blood.
I am very concerned that the diet that you are feeding Buster is deficient in calcium and many trace minerals and electrolytes. It is very difficult to design a balanced food for dogs. If you want to try making food for him at home I recommend working with a veterinary nutritionist to make sure you are feeding a balanced diet. Here is a link to a web page that will help you find someone to work with: http://www.aavn.org/site/view/58440_NutritionResources.pml
There is a company listed on that page called BalanceIt that formulates dietary supplements for homemade diets which will design a diet recipe for Buster based upon his history and likes. Many owners have been quite happy with their services.
In the meantime I do recommend that Buster see a veterinarian and have some blood tests done to look for organ damage and nutritional deficiencies.
Best of luck with your fellow, please let me know if you have any further questions.
Thank you for your quick reply. I can't imagine how he could've gotten organ damage. Are you suggesting it may be from his diet?