Does Glandular Fever Occur Twice And What Are The Tests To Be Done?

Is it possible for a child to get glandular fever twice? My son had it at 7 and now at 12 he has been sick for three weeks, intitally with flu like symptoms but now extreme lethargy and complaining of dizziness. His doctor said it was most likely a virus because we are in the middle of flu season. But three weeks is an awfully long time and he has not been able to attend school for any more than 2 hours at a time? He is an active boy, but tires naturally very easily. I describe him like a full tank at the beginning of the day and he is running on empty by 2pm. Always. He has an autism spectrum disorder (mild). He had blood tests this morning for glandular - but if if EBV stays in your system for life, how can they tell if a relapse? What else could they test for?