Student or Learner
He was so dreadful at the voice that he couldn't even move
He was so scared at the voice that he couldn't even move
The bold one is the answer my teacher provided me with (this is a fill-in-the-blank question) but I think dreadful at can still be used. So is it alright to put dreadful before at?
"Dread" is the emotion you feel when you are very unhappy about something that you know is coming.
You can be "full of dread" but that is not at all the same as being "dreadful."
Your original sentence could have been "He was so full of dread when he heard the voice that..."
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.