I, too, am curious if it is possible to train a non-aggressive dog to become protective of family and property if he/she doesn't come by it naturally.... not that I would ever want to do it. I hope the OP can resolve the issue in a way that benefits the dog.
Dog to Human. "Why do you want me to bark at that person? Are you crazy? I Don't get it. OK, show me a perp and I will bark my ass off for you."
You are happy to see the person coming in the yard, and your dog is doing the proper thing by taking his cues from you.
Perhaps when he's older he'll bark more but I bet he continues to look to you to determine if this guy is a bad guy or not!
You don't want your dog deciding, on his own, that Joe, your best friend, is a bad guy, trust me!