Given the task definition and input, reply with output. In this task, you're given a story (which contains five sentences only). Your task is to find all the characters which are available in the given story.

Sentence1: Alex had a young son. Sentence2: The boy was three years old. Sentence3: He still wasn't speaking. Sentence4: Alex told the doctor about his speech issue. Sentence5: Alex's son started to see a speech therapist.
Speech therapist, Alex, Young son, Doctor