Hi,
Sorry for taking too long to answer your question. I am not an expert on this topic and i might not have the exact answer for this problem. I have an antibody in my blood too. The doctors explained that this can happen because i have been transfusing for such a long time. Even though the cross matching of the blood is done to ensure the right blood being transfused, there is a small chance of the blood not being the exact match to the type of blood you have. After a long time, your body will produce some sort of antibody in the blood.
When there is an antibody surely it gets harder for cross matching. My hospital usually takes two weeks to find my type of blood because of the antibody. You can ask your doctor for the type of antibody your son have.
hope this helps.