Hi, My Son Has Gotten Very Tired In The Last

Hi, my son has gotten very tired in the last two weeks. He is an active football player and rugby. Suddenly, he had what seemed to be apendix problems. They rushed him into the er. Did a ct scan and an ultrasound. It showed swollen lymph nodes near his intestines. He had no diarehha. More of constipation. They said his white cell count was at 11,000, left sided. But everything else was normal. Probably a virus they said. He was sweating profusely at night, and had a 104 fever for four days. His fever has subsided.But he is pale, feels weak. Gets some dizzy spells, feels like he needs to sit after about 10 min. He did loose ten lbs, from not eating had little appetite. I am worried it is something like leukemia. Should I be concerned. Also, his knees hurt in the joints.