I don't know if you live near a PETsMART or not, but if you do, they offer a pretty good training class for puppies, and another for dogs. Whether or not that is an option, I can give you a little advice on my experiences with my Great Dane. The things my wife and I learned about dogs and their behavior, is that as pack animals, they actually do not want to be the leader, so you've got to make sure they understand that YOU are in control, otherwise they won't respect your commands.
Another thing is that they are so pack-oriented, that they need to constantly be told what to do to keep occupied, otherwise they resort to biting and chewing on things as a sign of being bored. The training class I took at PETsMART gave several suggestions on how to deal with this.
The basic training principles they use is to reward good behavior. If the puppy is doing something wrong, you don't want to punish it directly, such as hitting it, because then it doesn't learn that what it is doing is wrong, but rather it learns to resent you for hurting it, and it will only respect your commands while you are around, such as laying on the couch when you leave the room.
As far as what to do about "negative" behaviors, such as chewing, in your case, they have stuff you can buy, like Grannicks' Bitter Apple, which is a spray you can use on things to deter chewing. It's completely non-toxic to the animal, but it tastes terrible if they attempt to put it in their mouth. This way the dog will think that it brought the result upon itself, rather than blaming and resenting you.
If you don't like this approach (Some people refuse to use it, even though it's harmless.), then you have to make sure that the dog has plenty of toys or things you designate that it is ALLOWED to chew on so that it doesn't chew on anything valuable. Also, you have to train the puppy to know the difference. When the puppy chews on something good, reward it and give it praise, using a pleasant, reassuring voice. If it starts mouthing something it shouldn't, use a kind of loud, short, terse negative sound. Perhaps something as simple as "Hey!" or "AAAT!" You gotta sound like a pack leader.
As far as keeping the dog from hurting your nephew by biting him depends on how you want to approach it. Dogs like to roughhouse with other dogs, and it sees humans as other dogs in its pack. We were discouraged from roughhousing altogether, because it puts you on the same level as the dog, and makes the dog think of you as a peer rather than a leader (Isn't canine psychology complex?). This is where I went wrong, because I love roughhousing with my dog. If you're okay with your nephew playing with the dog in such a way, understand that even though dogs can look and sound dangerous with their bouncing and snarling, they never intend to cause harm when they roughhouse. A lot of people don't understand this, and assume the dog is being vicious. If you've ever seen two dogs roughhousing, and one gets bit too hard, it will yelp. The other dog will realize it hurt its fellow pack member, and will not bite that hard again. The same goes for humans, since they see us as members of their pack. You can prevent the dog from ever biting anyone too hard by teaching it to bite gently. Using dog treats, try to give the puppy a treat the "wrong way" (with your palm-side down, holding it in your fingers so that the puppy can't really get the treat without biting you). When the puppy tries to bite the treat and makes contact with your hand, yank your hand away and yelp, still holding the treat, and repeat this without giving up the treat until the dog bites "gently". Oh, just a side note, you should use a soft treat, because hard, crunchy treats take time to chew, and by the time the dog is done with the treat, it will have forgotten what it did to get the treat (I think it's about a 3-6 second attention span.).
For other negative behaviors like jumping on people (something all dogs/puppies do out of instinct that has something to do with getting their mother's attention.), you don't want to pay attention to the dog at all, because it will associate any sort of a response as the desired result. All it is trying to do in this situation is gain attention.
Whew! Okay, I hope this helps you in your endeavors. Just remember that training puppies requires a lot of work, patience, and love.
