Suffering from symptoms of past virus?

My son caught Mononucleosis a year back. Before then, he was always really energetic, never looked tired, even when he was bored. Ever since he recovered, he hasn't really been at all the same, in fact sometimes it seems as if he's still sick. He's been going to bed at about 8-9 pretty much every day this year. Usually everyone else in the house is up for hours past the time he goes to bed. What might be the problem?