Darkest Fear by Harlan Coben

In Darkest Fear, Harlan Coben has an old girlfriend inform sports agent Myron Bolitar that he has a thirteen-year-old son. The boy has Fanconi anemia and will die unless he receives a bone marrow transport. Unfortunately, there is only one match. He has disappeared and possibly may be a serial killer. Myron is confronted by moral dilemmas such as can he harm innocent people to save his son and should he try to have another child with this married woman to create another potential donor.