My nephew missing too much school?

I can't speak for high schools in Canada but schools in the US would be concerned about the credits that students need to graduate and usually you get a certain number of credits for each cl. Absences can affect getting those credits. And yes, those absences are a lot but given the illnesses that he has had, that happens. Have you spoken to the school? H1N1 hit all schools hard this year with a lot of absences so the school won't be surprised with that. And if you have a doctor's note for the mono, the school will be familiar with kids who have it so that should help, too. He may have to attend summer school but again, that may be different in Canada. Your best answers will come from his school so how about starting there if you haven't already?