So the only way you or your husband
can show
type A
blood is if you have the genotypes A,A or A,
O. And the only way for someone to show type B blood is B,B or B,O. For your son to be B -one of you would have had to have a B to give him. Which means one of you would have to be either B or AB blood type.
Either your son isn't yours- switch at the hospital or (not wanting to infer anything) a different father than your husband. OR they messed up on the blood test.