If your husband or you was AB or B, then that would be possible your son is B.
I am thinking you are not correct about one of the blood types.
Generally speaking no. But if you are checking for whether or not the child is yours or someone elses then they have to do further testing because the ABO blood typing isn't enough. There could be an error as well. Your physician can refer you to a testing site.
Retest both yourself, your husband and your son at a reliable, quality assured lab. Parents with both A type blood will produce a child with A or O blood type blood.
No it's not. Blood has three main types-A,B, and O. These are typed by the sugars and antibodies on the blood themselves. Type O has none, so A or B would be dominant over it. (If a person get's a O allele from one parent and a B from another they would show type B blood; same is true with A). A and B are called "co-dominant" because they show both, one doesn't overpower the other. 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. To further investigate it you may have to do a paternity test to see if your son is really yours.