he is probably salivating and not eating because he feels as though he is going to be sick, and he is lethargic because of this too. offer plenty of water so he does not dehysrate and if he doesnt want to drink try wetting his lips/gums or syringing some into his mouth. offer him some warm chicken or heat some of his normal food up a little bit, cats like food that they can smell, and by warming the food up it might encourage him to eat. if he appears lethargic for another 24 hours see if a vet would do a home visit. just let him curl up somewhere warm if he doesnt want to be fussed. check he is going to the toilet, he wont pass much if any faeces if he is not eating but check for blood or mucous. hope this helps.