Can a o negative man father b positive child?

No, a O negative man cannot father a B positive child.

A child receives one allele for each gene from each parent. If the father is O negative, he can only pass on an O allele. The mother must therefore contribute the B allele. However, the mother cannot have B positive blood if her other allele is O, as this would make her genotype BO, which results in a B negative blood type.

Therefore, it is impossible for a O negative man to father a B positive child.