Why is my dog afraid of my dad?

Your dog has a fearful temperament and recognises your Dad as the alpha male. Get your Dad to feed him every day, He shouldn't make direct eye contact or try to pet him.
Its nothing your Dad has done.

You arent afraid of him.

Some dogs are intimidated by men. And it could be nothing more than the timber of his voice. It sounds crazy but animals like dogs and cats are drawn to women because we usually talk to them in "baby talk" and that helps them feel secure. Have your dad try it. He may feel ridiculous but trust me it works!

If your dad is loud & gruff, too stern, uses wrong corrections, yells too much it is only natural for a dog to accept that your dad if Alpha in your family. His personality may be too strong to a dog that is more submissive than dominate. It could be your dad or it could be the dog or both.

The best person to ask would be your dad

What are the signs that makes you think he's scared of him?

Maybe he's just scared because he doesn't know your dad very much. Try letting your dad give her food, or pet her. Let him spend more time with your dog, and your dog will know him better and shouldn't be scared of him anymore.

It could be that your dad is having sexual affairs with you're dog which is very common.