Answer:
In autosomal disease, mutation occurs in any gene present in 22 non- sex chromosomes that is called autosomal chromosome. In autosomal dominant disease inheritance of mutated gene from only one parent is enough to cause the disease.
Therefore if a child has an autosomal dominant disease then one parent must be affected with that disease and in this case, the father does not have the disease so the mother must have the disease if his son is affected with the autosomal dominant disease.
So it can be deduced that the boy's mother must have the autosomal dominant disease.