My guess is Donnie's muteness may be far more complicated.
The boy seemed attentive in spite of his muteness so Dean resumed sweeping up needles with his hand and continued to chatter.
Did the doctor believe Donnie's muteness went back to the death of his half-brother?
The weak parts of this story are the sudden and unexplained departure of the Simons; the subsequent useless cruelty of treating the child like a wild beast and keeping him in a dark room practically out of sight (unless any doubt of his identity was possible), while his sister was in comparative comfort; the cause of death, declared to be of long standing, but in fact developed with such rapidity; the insufficient excuse provided for the child's muteness under Gomin's regime (he had answered Barras) and the irregularities in the formalities in attending the death and the funeral, when a simple identification of the body by Marie Therese would have prevented any question of resuscitated dauphins.