Mono is not considered a "serious" viral infection. To me at least, that's reserved for life-threatening. If it's just mono he could still be out more than a couple of weeks, depending on how he recovers. Spleen gets enlarged during a mono infection so he would be no contact for at least a couple of weeks.
Edit: Just looked it up on UpToDate, splenic enlargement happens in more than 50% of mono infections and usually starts receding by the beginning of the 3rd week. Recommendations for strenous contact sports, no earlier than 21 days after the onset of symptoms. So 3 weeks if it's mono.
Edited by nooneLT, 01 August 2013 - 04:38 PM.