I would be reporting the breeder by the way, not only have they sold you a sickly kitten (which I think is against breeders guidelines), but to be sold a cat that has been fed an inadequate diet. I would consider getting some kitten milk replacer or Hills a/d from the vets, and seeing if that helps, as if he has only had tuna after mum's milk, it will make it harder to get better, as it doesn't contiain everything they need. IF he has a runny nose, he wont want to eat as he wont be able to smell, maybe heating the food up will help. Good luck with him.