I would imagine that the 15 year old son knew that he had a monster for a father and feared what would happen to him.
Give the kid a break. That whole household was probably terrorized.
He could be far more afraid for his siblings. It is very easy for a father to scare a kid to death, especially if the father demonstrates torture on a daily basis. An adult would go straight to the police and won’t return without cops. But a child probably witnessed the events growing slowly, step by step, sold to him as something acceptable. Another possible tactics for the father is to claim that the police won’t believe a minor (which, unfortunately, is possible.) Imagine what could happen if the child talks to the police and not believed. An idiot cop could call the father, receive assurances that all is just peachy and send the kid home. That would be a death sentence. Same applies to the 19 y.o. daughter, though she is old enough the know better; but it’s for the judge to decide. Scaring her off was not a bright idea.