Whatever you do, don’t do what my sister has done….”its o.k. boy, don’t be scared baby”, all the while petting it and giving it positive reinforcement for behavior that you do not want.
My sister has a dog with some of the same problems. She had him in obedience and they worked on his fear, and wanted to curtail his behavior from getting into full blown fear aggression. She did a great job…except he is her baby and she does not see that she is NOT helping the dog with trying to console the dog when this behavior is occuring. The best behavior on your part is stay calm, assertive, and don’t feed into his aversive behavior. Yes, see a trainer, but your reactions are the most powerful here.