Fever and swelling near neck. To my son.

My son,aged 3 years has been suffering from fever since yesterday mrng...the fever is very high going upto 103.5..we gave p-250 syrup till yesterday evng ...This is 4 th time in a month...10 days back  local doctor advised 4 to 5 blood tests,urine test including dengue test...everything found to be normal.yesterday At around 8pm..my son complained about pain near neck..I observed it and found swelling there..we hurried to a nearby doctor fearing it to be mumps...he gave some medicines and asked to use for 3 days..he didn't conclude it to be mumps or tonsils..we are very much worried about his health...falling continuously ill..becoming weak and uninterested to eat food...plz advise us ...what to do and what could be reason for this illness...till 2 months back..he is absolutely fine..what happened to my son..plz help..