Hello and thank you for your question.
What you are describing are very nonspecific symptoms for illness. It may be related to having a fever, or infection, esp if he was in a fight or has an abscess... It may also be related to a urinary problem... esp in males, we worry about urinary obstructions that cause them to be lethargic, uncomfortable, and unable to urinate properly. This is an emergency situation and should be checked out immediately to rule these potential problems out. It is possible if he ate something that has upset his stomach, he could be lethargic and ill from that, too, and will likely need some medication for nausea and antibiotics to help him recover. If possible, I suggest you have a vet examine him to help rule out the potential emergency conditions and then you can make a treatment plan from there.
I hope this is helpful and please let me know if you have any specific questions.
Cat Veterinarian
10+ years practice experience as a cat/dog veterinarian- special interest in behavior