If your brother really hits the puppy, I doubt your dog will need "training" to learn to not trust him. Do not allow this to keep happening. Keep your brother far away from your dog. That is unacceptable, and you should not allow this to happen, period. Your dog is old enough at this point to be able to hold it until you get home.
Forrest - GSD 9/1/2016 - 5/14/2017 RIP
Brooklyn - Golden retriever 1/30/11