I guess it depends. Has it started recently or has he always had it? As you most likely know, the further you get into masters and doctorate level work the more demanding it can be and he may be just having a hard time with all the pressure. Also higher level education starts to demand you make assertions instead of just taking notes and remembering what you were told (damnable thesis work). It could be anxiety or could be ADD or something else. Maybe he could talk to someone and get evaluated? Good luck. I wish him well on his masters work.