Dog Scared of certain people

Question
I just adopted a one year old black lab/mix from our local animal rescue about 5 months ago. He is well behaved, mild manner, and very sociable. He loves attention! For some reason though he is terrified of my 6'4'' brother. My brother is very kind to him, but my dog urinates everywhere whenever my brother pets him, or even enters the room. He is so afraid of him sometimes, and I am confused as to why. My father and grandfather are both about the same height as my brother and my dog adores them. Do you think my brother reminds my dog of someone from his past home? What can we do to fix this?

Answer
I encourage people to adopt and will do whatever I can to help with past baggage and other problems.  Have your brother offer it treats.  He needs to be careful to not make eye contact or let his teeth show by smiling.  He should keep his hand away from the top of dog's head.  A butt or belly rub is much more welcome than a pat on the head.  Control your emotions.  If he senses your stress, that will confirm your brother is a problem.  Instead greet him warmly and keep up a happy chatter.