I am a single mother, I recently became pregnant with a mans baby but we split and my ex boyfriend and I got back together and he wants to marry me and claim the child in the state of Illinois. Will he legally be the father

Illinois Paternity Law - Paternity Laws - Paternity

Under Section 5 If you marry and he signs and acknowledgment of paternity that you agree to, then yes he would be the legal father. However, the natural father can challenge.