Question

a man has type AB blood is it possible for him to an O child

Answer 1

no it is not possible if mother have o then children may be

Answer 2

An O and B crossing can not produce an A or AB child. An AB with an O can produce A children or B children but not O. In short the ABO system can prove you are not a parent but not that you are, as there are millions of other people with the same blood group.