Bad Dog

I have a possessive angry little dog. He will shy away from everyone but when far enough away from them turns around and comes attacking. He only likes one person that lives in the house, shows aggression to everyone else and barks at anyone walking into the house he is about 2 or so. We got him from a shelter 1-1/2 years ago. He has been this way since we got him but seems to be worse now. He has spells at times when he will just growl and show his teeth at other dogs, people, anything that moves, cars going by on the road, the wind, just about everything sets him off. He has a yard to run in and can go in and out anytime. What can I do to help get this dog to act better. If he gets worse I'm afraid he will need to be put down.