What exactly are you calling aggressive behavior? You'll need to elaborate since folks have a wide variety of what they feel is aggression.
Do you know his breed for fact?
Is the fence a private fence or see through? I honestly would never keep a dog in a fenced yard unattended I prefer kennels and crates. Prevents any bad situations and annoying neighbors from messing with the dogs.
It's normal for dogs to alert when people come onto their property "Hey! Mom! Someone's here!"
If you want him to be less reactive to guests, this could be helpful Look at That! A Counterintuitive Approach to Dealing with Reactive Dogs Dog Training for Dog Lovers Blog