My son's speech therapist has helped him improve significantly.

therapist 是由 therapy（治疗）加上后缀 -ist（表示"从事某种职业的人"）构成的名词，指"治疗师、治疗专家"。 speech therapist 指专门治疗言语障碍的专业人士，即"言语治疗师"。