That's pretty much what I have been doing. He is fairly neutral to people most of the time. When I hesitate to correct him is when he really gets going (like last night). There was someone that he doesn't know that showed up on my driveway after dark, so I wouldn't expect him to be comfortable with him. And he needs a pretty solid correction for it to sink in, especially once he takes on the role of "protecter", so the last thing I want to do is make him even more stressed about the situation. If I need to be harder on him, I'm willing, but he's not even 6 months old yet (wouldn't know it, he's well over 50 pounds).

Going by the description you've given of your dog, the behavior he displayed was very predictable and to a large degree, he was doing what he should do. What you might have done is have the dog on leash and then invite your friend closer to the house or inside the house and use a command to let the dog everything is okay. Then sit down for a while and keep your dog calm. This way he learns that if someone comes around the property ( I'm assuming you have a fence and your dog was not totally free) he can show some defensive behavior, but when you allow them to come in, he can stay by you but must remain calm. It sounds like you didn't have a leash on your dog, so there was no way for you to manage the situation. Allowing him to bark at your friend for five minutes was not the best way to handle this situation. The pup needs more guidance from you. Have your friend come over again and try your pup on a leash and teach him to sit calmly beside you while you friend visits. I don't think it is a matter of being harder on your dog, but rather a matter of being smarter in anticipating your dog's behavior and having a plan to deal with the behavior and shape it in the direction you want.

Simon,
I might suggest that you surf the web and find a breeder/owner/handler of working Bouviers (William Kachman comes to mind). Maybe e-mail and pose your question to them. I read an article by Kachman or Fokkja Krotcha (spelling's wrong) several years ago and he talked about the differences between Malinois and Bouviers. He addressed the aggression differences and temperment differences between the two. Just something you might consider.
Randall

That seems like a good place to start. It's been tough to get a good evaluation from those that I know, because they all use shepherds and mals; we don't even get to his nerves because they wonder why you'd train a dog that isn't jumping off the walls with prey drive (I'm starting to wonder that myself...).

Again, I appreciate everyone who's posted, you've been very helpful.

Any advice on how far back I should push back any defensive element to his training when he's naturally so defensive? Obviously a dog like this should be worked in only prey for much longer. My problem is that his prey drive isn't fantastic, and my experience with another low prey bouve (now about 13 months old) showed that adding some defense brought out not only his aggression, but also seemed to load up his prey drive (just like Kevin said it might, thanks...),

Just try to not put him in situations that are likely to trigger a defensive reaction, and if he does get defensive when you prefer he didn't just move him away from the situation with a pop on the leash and distract him to something else without bringing a lot of attention to the situation. You will not be able to "push back" his defensiveness, but you can try to keep him out of situations that are likely to elicit a defensive reaction until he has matured more and hopefully is more comfortable handling the stress he feels when he perceives something as a threat. His defensiveness will be there later when you want to tap into it.
What are you doing with him in the bitework? Are you working him or do you have a helper? Are you using a tug in the bitework? If so, having the tug attached to a leash allows the helper to distance himself some from the dog when he is on the tug so the dog is less stressed. Then the helper can walk his way up the leash and get the dog more comfortable with helper getting closer. If he won't work in prey at all, you can try to channel his initial aggressive/defensive response to the helper into a prey bite so he can relax more in the bitework.

Right now I do most of the bite development with him. Earlier I had gotten a bit ahead of myself, but I have him working more and more confidently now on a puppy tug. I'll probably have my future helper start doing light tug work with him soon, as I'm hoping to introduce him to a helper before he reaches maturity (I figure any aggression now is only likely to escalate then, and I definitely want him working as comfortably in prey as I can).

The advice to re-introduce a leash attached to the prey item when he starts on a helper seems like a good plan. He'll play tug with those that he is really comfortable with, but if he doesn't know them well, he'll often back off and bark (definitely not a prey bark).