What Causes High Fever And Low Appetite In A Toddler?

hi I have a 2year old with a 42 degree temperature. has had a high temp for the last 36 hrs and has lost his normally voracious appetite. he is very quiet and lethargic. he still drinks. i tried to breastfeed him but he wretched as if he was going to vomit after a few mins. i have given him baby paracetemol ( calpol) 5ml every four to 6 hours but the temp is still there. should i take him to see a a doctor? or wait?