What causes a persistent, unexplained high fever in a four year old?

My son has had a high fever (between 102-104) since March. He has it for a couple weeks, then it goes away for a couple weeks, then it comes back. I have taken him to the doctor numerous times and they can't figure out what is wrong. There are no other symptoms. No cold, flu, strep, urine is fine, etc. They have done blood work and still haven't come up with anything. Please does anyone have any suggestions or anything that would help or that I could research. Anything!! I am planning on taking him to another doctor for a second opinion soon but I need to figure this out. Any help is appreciated greatly.