What Causes Chronic Headaches Along With Fatigue In An Adolescent?

My son is 15 years old and has a history of Ulcerative colitis which flares occasionally. Three weeks ago after a few days after a UC flare had subsided, he presented with a severe headache (8/10 for severity) that has since reduced to moderate in severity (4/10) but has not subsided for 16 days. He is also very fatigued and has glossy eyes like he is feverish although his temperature is normal but has gone up to 99.1 a couple times in the past 16 days. His UC has been under control since the onset of these headaches. He has normal blood tests and normal CT scan of the head. We are now checking for Lyme disease although he has no recollection of being bitten by a tick. Any idea what is causing these debilitation headaches?