Why Is My Toddler Under Fever Every Alternate Month?

Hi, My son is 3 years old. From this last august he is repeatedly having fever every alternative month...and that fever remains for 10-12 days. Every time he is being given a antibiotic, fever medicine and an anti allergic. all the tests have been done 2 times. now again he is having fever from 29th jan and from that date he is been given antibiotic. Kindly, suggest what is the cause and what should I do.