What Causes Loss Of Appetite, High Fever, Excessive Sleepiness In Children?

My 14 year old grandson has been to the doctor 1 and ER 1 in 7 weeks...he is running a fever between 99. and 104. Blood work was done they said it was just allergies to take Zertec....He is not hungry, he does not feel good, he sleeps a lot...this is a young man that is a 4.0 he never says he is too sick to go to school but he has missed at least 9 days this last 7 weeks....I am thinking rare cancer, Scarlet Fever.....etc. please help. Valerie