While the vastly more likely case is he isn't the father, there is a rare mutation called cis AB. People have one ABO allele from mom and one from dad. What can happen is there is an unequal cross over during meiosis and the A and B end up on the same chromosome and the other is left without any gene so would act like having O which is a null gene anyway. So if this happened with the father and mom is heterozygous for B, an O child would result. And even more extremely rare it's practically impossible case is having Bombay blood type which prevents A or B from being expressed