Ann
It is possible for a Type'O' parent to have a Type'A'child, as long as the other parent tests as Type'A'. Each parent has 2 genes, and passes 1 of the genes to the children. Type'A' parent has either genotype AA (homozygous), or AO(heterozygous), and Type'O' parent only can be homozygous genotype OO, because Type'O'is a recessive gene which needs 2 'O' genes to be expressed. Since you typed as an 'O', and can only give an 'O'gene, we assume your husband is heterozygous for Type'A' blood type. Or, your son's blood test needs to be repeated.