Welcome to healthcare magic. Infectious mononucleosis
had a self limiting course of around 4 weeks. You could repeat the blood titres ( Paul bunnel test) to see if it is coming down or rising compared to the previous values. But the post infectious malaise
are well known to persist for a few more weeks following full recovery from mono. Make sure he has a good nutritious diet, takes lits of fluids, adequate rest. He could also take multivitamin
capsules for a few weeks.
Hope this helps you.