Without further information it is very difficult for me to provide the best advice but I shall certainly try.
Please reply when you can and we can continue the conversation.
A reduced appetite and lethargy in a young cat can have a wide range of causes including trauma, a cat bite abscess, toxin ingestion, pancreatitis, a viral infection etc.
A vet visit is best do he can be examined. They can check for any source of pain, assess his hydration and determine whether or not he has a fever.
If concerned, they may advise some tests such as a blood test or imaging.
Treatment will depend on what is going on and may include e.g antibiotics or anti inflammatories.
Try to tempt him with warm food such as chicken and fish.
Double check the home for any potential toxins such as a houseplant, human medicine or cleaning product.
Hopefully he is on the mend very soon.