Since I had never seen her son, on theoretical grounds, I conjectured that his stuttering could be a reaction to the fear of father during the oedipal phase.
She commented that her son has not known his father at all. He had left the family soon after the child was born, and has not maintained any contact with the child. She herself had been quite irresponsible, indulging in drugs, when the child was in his Oedipal period. It is her brother, who is a fireman, and is stable and responsible, who took over the boy's parenting. But he is not strict at all. So she does not understand why the child would have fear of his uncle (the father figure).
Mother agreed with that and said that despite my brother not being authoritarian, my son does show shyness and fear around him. And she will tell what I have said to her brother.
A couple of months later, the patient informed me that she did have a talk with her brother who approached the boy and told that behind the stuttering lies a fear of him.