because he isn't trusting you to do it. He just needs to be taught that this behavior is not acceptable. Be the leader. Whenever he starts to do this behavior correct him. The second he starts to do whatever he does in this territorial behavior (barking, running toward the person or whatever it is) physically stop him. This can be done with or without a leash, but I've found in the beginning correcting is easier with the dog on leash especially with larger dogs. Snap the leash to the side toward you to divert his attention... this is a sharp tug and release with the same intensity as he's showing. In other words, the more intense he is, the sharper the tug otherwise it won't be enough to divert his attention. Dogs can't give their undivided attention to more than one thing at a time (this is part of what makes them so easily trainable). Ideally, he'll stop and look up at you like "What?". If he doesn't pay you any mind and just goes right back to the bad behavior, do it again more sharply. You want him to stop and look at you for direction. The second you have his attention, make him sit and be quiet so he can learn to greet and be greeted properly.
Since this behavior is very new, he probably won't be difficult to re-direct... it's more difficult with dogs that have a long habit of being allowed to do a bad behavior. However, dogs are quick learners (far more so than people) and dogs that have done a certain bad behavior for years can learn to behave differently with just a few corrections.
Consistency in all things is the big key. You must, must, must correct every single time. Dogs - unlike people - won't follow an unstable leader, and only consistent leaders are stable in a dog's world.
Voice commands other than the very basics like no, sit, stay, come, down, etc. are really unnecessary and may even cause confusion in the dog. Words have no meaning to a dog, but sounds do particularly one syllable sounds, and tone is important. However, dogs respond much better to body language since that's mostly how they communicate. For the most part, I only use a few words and one particular sound that works with my dog to get his attention, and everything else is gestures and body language. I've found he responds much better to body language commands than with voice commands.
You might find a lot of these video clips helpful for just the problem you're having... how to stop the dog's bad behavior and get his attention, how to tell the dog what you want him to do, how to meet and greet people and dogs properly in the house, yard and out in public, etc...
http://channel.nationalgeographic.com/channel/videos/player.html?channel=39678Right off the top of my head, there's one video clip called "Pug and the Pig" in the Disobedience category that shows how to teach the dog to let people come into the house and wait to be greeted. Yes, this owner has a little pug doggie and a great big fat pig that appears to just live in the house like a dog. I know I've watched a lot of other ones similar that include really bad behaving dogs, great big dogs and whole packs of dogs being taught to let people come in the house and wait to be greeted and how they greet people with good manners. If you click on the little link near the bottom that says "view descriptions" you can find which videos would be the most beneficial for your particular issue.
The full episodes at the website are even more beneficial for real problem issues and really stubborn really badly behaving dogs. One of the things I think is really maybe the most beneficial about these videos is they show not just the expert doing the correcting but the owners learning to do it themselves and being corrected by the expert in whatever it is they aren't doing right. Ok, and a lot of the vids are fun to watch just to see how truly awful some of these dogs are and how easy their bad behavior can be turned around.
From the brief description you gave, I think this isn't fear or aggression or fear/aggression but mostly territorial with probably some protectionism, too. The bad part is that territorialism and protectionism if left uncorrected can turn into aggression, so it's good that you see this behavior as a problem and are wanting to nip it in the bud, and that's half the battle. The good thing about his exhibiting this behavior after only a couple of weeks with his new family is that he's accepted his new home and people and is comfortable and happy now in his new home. That's a pretty short period of time, so it's good that your new family member has already adjusted so well, which is a really good sign for the future. Of course, the behavior itself isn't good, but it's a positive sign that he's adjusted so well and so quickly.