Hi,
It is vitally important to have the respect of your dog. By hitting him your husband has no chance what so ever of gaining any respect at all. Unless it is an incredibly stupid dog it will avoid contact with your husband whenever it can. As soon as he makes a demand that he thinks the dog should obey he is instantly sending a message to the dog that says, "I'm talking like this and looking at you and that means I'm going to hurt you." Your husband has trained the dog to avoid him. If he persists in this method the dog will try harder and harder to avoid contact with him and many small dogs resort to fear biting to avoid the pain of contact with the human hand. We have all the see the snarling little yorkies snapping at anyone that tries to touch them.
Your husband could turn this little dog around in a day. But he needs training first! A dog can not obey a command that it does not understand. Your dog has been taught that "Come" means that it is going to be hit. What I am saying is that your poor little chap has no idea what your husband expects of him and no idea why this enormous being is bullying him for no reason. Put in that situation I would pee myself on a regular basis too!
Training a dog MUST start with positive reinforcement only! You can only praise and reward the dog when it does something right and never punish it in ANY way when it makes a mistake. This is the only way for the dog to learn what you want it to do! Happy excited voices, crouching down and offering tasty morsels will teach the dog that when your husband says, "Come" it is a wise and rewarding experience to go to him. This must happen over and over and over, always a good move to go to the big man when he says,"come". Only when your dog has this concept ingrained in his head is it even possible to show displeasure when he fails to obey and expect the dog to understand the reason for it. To show displeasure should never, NEVER, mean hitting the dog. It doesn't help anyone and will destroy any respect the dog has built for you. Show your displeasure by saying the dogs name in a very stern voice, not loud, stern. As soon as he looks at you revert to the happy voice and repeat the command. The trick is to make yourself more interesting, welcoming and more pleasurable than anything else the dog may want to do. Never lose your temper, never give in once a command has been given and never hit a dog. If your husband cannot change his attitude to the dog then the only fair thing to do is to find a dog friendly home for your dog or a dog free home for your husband. He is abusing this animal and it must stop. Ignorance of training principles is no excuse for a grown man who is abusing a Yorkshire Terrier. The humans in the house should be the top dogs and that means fair and consistent handling of a dog that knows the rules. Rehoming your dog will free it from the constant anxiety, confusion,abuse and fear it is living with at the moment. Thank God he didn't get you a Rottie or Dobbie with the mental strength to stand up for itself!