It could be mono or pneumonia. My dd was very tired when she had pneumonia and mono is spread like a common cold but sometimes takes a big toll on some people. If he isn't eating or drinking much that also contributes to lethargy. If yoy think his lack of appetite is the problem i suggest getting some sugary juice and pudding into him fast and seeing if that helps. My dd's pediatrician suggested this when dd had mo sugar in her urine and was leathargic snd it made a difference very quickly. I suggest having him seen by a doctor ASAP if he doesn't improve quickly.