My Son Had Flu Symptoms The Middle Of August (stomach

My son had flu symptoms the middle of August (stomach ache, nausea, headache, sore muscles) for about 4 days. These went away but he has been feeling very fatigued (but can't sleep), headache that never goes away, and muscle soreness. He is 15 and very athletic. He can sprint a half mile without hardly getting tired and has never had sore muscles even when starting a new sport. Right now he can hardly get up the stairs without panting and his muscles ache all the time. I have taken him in several time to the doctor. They did several blood test and told us he had mono on August 31st. He hasn't improved any, if not worse, so I took him in again yesterday only to be told it was a false positive and he does not have mono as the Epstein Bar Panel was relooked at. We were told that he just needs to wait 2-3 more months to see if he feels better! I am very concerned because something is not right.