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It is certainly possible for fleas to cause a young kitten to be lethargic. Fleas take a blood meal and, especially if there are a lot of them, can cause anemia in small, young cats. Anemia, in turn, can make a kitten lethargic.
If Kemik had fleas on him, then you have fleas in your house. So flea combing is helpful, but the ones in the environment will jump on him as the larvae turn into adults.
Young kittens can also develop upper respiratory infections, have intestinal worms, and eat things they shouldn't, which could all cause some lethargy (in addition to other clinical symptoms). So if you haven't taken him in to a regular vet yet, now would be a good time.
But I would definitely get Frontline or Advantage to put on him. Both of these topical medications can be applied to kittens 8 weeks and older, so Kemik is not too young. But do use a well-known brand name. I find products by Hartz or other OTC brands to be hard on kittens and not as effective. It is important to treat all the pets in the household, as they will all have fleas.
Hope that helps!