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From the symptoms you describe, it does sound like Mr. Boo is sick. It may be that he has a fever. The primary symptoms indicating a pet has a fever is lethargy and poor appetite. Most of the time in a pet this is due to some sort of infection. In this case it may be that he is having a gastrointestinal tract infection or inflammation causing his symptoms. I think it would warrant taking him in since most of the time antibiotics are needed to clear up the problem and to get his energy and appetite restored.
Please let me know if you have more questions,