my axolotl seems sick and idk what to do

i’ve had my axolotl for about 6 months now and i went on a holiday for a few weeks and had someone look after him while i was gone. once i came home he seemed to not want to eat as much. fast forward about 2 months he’s still not really eating much and his gills are starting to rapidly disappear, he’s very skinny and still not eating much no matter how much i feed him, he’s also changed to a more white colour, any advice on how often i should change his water or feed him would be greatly appreciated