My first thought is HFMD (Hand, Foot, & Mouth Disease). I swear it catches me off guard every time (I have 4 children). Check the bottom of his feet, his palms, and his mouth for tiny red spots or blisters. One would think you'd notice them, but I never did until I purposely looked for them b/c they can be small! Also, sometimes the "blisters" might congregate more in one area vs. another. For example, when my daughter got it she could barely walk b/c it hurt so much. When my son got it he could barely swallow from the pain. But the one common theme was exactly what you describe. High fever followed by being clingy and sad and lethargic. If it's not HFMD, it definitely sounds like a virus. :o( Stinkin' viruses!