My kitten has no energy?

We have a selkirk rex kitten who is 16 weeks old. He is eating and drinking but just seems to have no energy. He is not interested in any of his toys and just seems to have no life to him. He is up to date with his vaccinations, has been wormed and treated for fleas. I'm wary about taking him to the vets as last time I thought my dog was unwell I was charged a very expensive consultation fee just to be told there was nothing wrong with her.