What Causes Persistent Fever And Fatigue In Children?

My son is 2 1/2. He has had an unexplained fever for 4 weeks. Comes and goes through out the day it seems to go down when he sleeps and then rises back up through out the day when he's awake. No other symptoms besides fatigue. He has lots of blood work, cheat X-ray and sinuses X-ray. All blood work normal. Sinuses X-ray showed possible sinuses infection out on antibiotics for 10 days and nothing.