What can I do for my dog who is aggressive when he is scared?

Usually this occurs at home when someone lights a firework in the neighborhood, or if he sees people he doesn't know. He is a terrier mix and 27 pounds. He is 5 years old. Most of the time he is a wonderful dog but he is only comfortable with certain people. He is still a little iffy around my brother in law and nephews and has bitten them a few times. He gets in weird moods where he is territorial also. If he is under the bed, he growls at anyone who tries to get him out. Also, if someone walks to the bathroom, he runs to our room which is right next to it and growls. He isn't like this with me, my husband or my parents. Also, he has never been aggressive towards the vet, groomer, or when I leave him at a kennel when I go on vacation. If he is panicking because of a firework or some other reason, he often snaps at my husband or anyone who is near him. Anytime someone comes over, I have to put him in my room because I'm worried he will bite. He looks interested and really friendly but as soon as they either stand up or get to close, he growls or snaps. How can I teach him not to bite out of fear? Since he isn't like this often, I don't know that obedience school would help. We may not be able to simulate the examples since many times they are unexpected.