My 8 Year Old Son Has A Stiff Neck On

My 8 year old son has a stiff neck on his leftside, cough and a slight fever. 99.9. the entire house has had sinusitis for two weeks. he hasn't gotten it. But now three days with a stiff neck. He did receive a flu shot in December. Our middle son is under chemotherapy treatment for Acute lymphoblastic leukemia since 2014.