Friend, I am sorry that you are having this problem with your dog. In response to your question, lethargy and decrease appetite are very non-specific symptoms and can be related to many different medical conditions that affect dogs. Unfortunately, on history only it will not be possible to determine the exact underlying cause, I am sorry. But, common possible causes could be anemia, infections, ingestion of a foreign body, kidney or liver disease, etc. to name a few causes.
In this case, I would strongly recommend not waiting any longer and have your dog examined by a veterinarian. You see, there are medical conditions that if caught early carry a better prognosis and less expensive to treat. For this reason, it is best to not wait. There is really not much I can recommend for you to try at home that would have any significant therapeutic benefit, I am sorry. I realize that this answer may not be entirely to your liking, and I regret being the bearer of information that you really don’t want to hear. But it would be unfair to you and unprofessional of me were I to provide you with anything less than truthful and honest information. I hope you understand.
My goal is to provide you with excellent service - I hope I've provided the information you were seeking, if you feel you have received anything less, please reply back as I am happy to address follow-up questions. Kindly rate me when you are done.