You need to give positive reinforcement when he does the behaviour that you want to get out of him. For example, my 7 month old boxer is starting to get protective of the house. When people walk by the front yard he jumps up to the windows and starts to bark under his breath and growl a bit. Whenever I see this happen, as soon as he jumps up all alert, I say "Who's there?". Soon it will get to the point where whenever I say "Who's there?", he'll run to the window to look outside.
Hope this helps.