Infectious mononucleosis is a mild viral illness which produces fever, fatigue, sore throat, and swollen lymph nodes. It can cause liver inflammation (hepatitis) and enlargement of the spleen and in your case due to poor appetite the child has lost weight,which he is likely to regain once he starts recuperating.In most cases of mononucleosis, no specific treatment is necessary. The illness is usually self-limited and passes much the way other common viral illnesses resolve. Fortunately serious complications associated with Infectious mononucleosis like hemolytic anemia, pericarditis, lymphomas certain cancers are very rare.