Well A and B are the dominant types, over O, and you have two alleles each. A and B are also jointly dominant. This means that if your blood type is A, your alleles could be either (A and O) or (A and A)
If your blood type is B, your alleles could be either (B and O) or (B and B)
If your blood is type AB your alleles will be A and B
If your blood type is O your alleles will be O and O
Therefore you must have OO and your husband has AB, so your child should therefore have AO or BO, therefore making him bloodgroup of either A or B