What can i do if my son has an anc of zero and is currently in the hospital with low fever?

Many possibilities: Infections, especially certain viruses, are the most common cause and can suppress the body's ability to make neutrophils, resulting in a very low anc. Once the infection is cleared, the ANC will recover. Rarer causes of a low ANC can include autoimmune processes, genetic disorders, or processes that affect the bone marrow, such as aplastic anemia or leukemia. I hope he gets well soon.