Is this your first puppy, NicoRotty? At what age did you get him and do you know if he was handled extensively by the humans in his first home?
Biting and growling are pretty normal puppy behaviors...you'd have seen your pup do the same amongst his littermates. If you're as worried as you sound, I'd recommend you find a certified professional (not just a trainer)...you can contact a veterinary teaching hospital in your area for references. Your pup will be thoroughly assessed and based on the findings, you'll be given the appropriate behavior modification exercises to do with your pup.
Here's some info you may find helpful...
8. How can I stop my dog from snapping, biting at my clothes, or biting at my leg when I walk away?
This is a really bad habit that starts very early – it’s how puppies play. They run up to anything that moves and bite it, and we humans think it’s cute. When they get to be adolescents, however, this type of behavior is just not appreciated by people.
For some breeds, in fact, it’s instant damnation. People will think your pit bull or Rottweiler or shepherd is being aggressive. He’s probably not, but he is acting unmannerly, and in a fashion that’s downright dangerous for his health. All it takes is one person to report you have a dangerous dog and he becomes a legal entity. So regardless of his playful intentions, you must stop this.
So what do you do with this behavior? You say, “Stop!” Let him know this is inappropriate. There’s no need to shout or frighten the dog; there’s absolutely no need to hurt him. If necessary grab him by the collar, but only if necessary, as the last thing you want is your dog to develop a negative association with other people. Instead, praise your dog when he does act appropriately around people. Tell him, “Good boy! Yes, that’s the way to say hello!” It is also a good idea to give your dog a toy to carry in his mouth so he’s busy and less likely to try this behavior.
If you do have to reprimand the dog for lunging, snapping, or getting hold of people’s clothing, afterward cue the dog to sit. Then, ask the person if he wouldn’t mind giving the dog a treat. You can now have the dog approach the person in a mannerly fashion and sit, at which point the person gives him a treat. The dog learns he can say hello, as long as he does it in a fashion which is acceptable to humans.
9. How do I teach my puppy to stop biting and nipping?
Most puppies are virtual biting machines with needle-sharp tiny teeth, and they are going to grow up to be adult dogs with powerful jaws, so bite inhibition, or how to use their jaws gently, is the most important thing for them to learn.
Teach bite inhibition in two stages:
1. Limit the force of the puppy’s bite.
2. Inhibit the frequency.
It must be taught in this order; if you completely stop the puppy from biting too soon, he’ll never learn to inhibit the force of his bites. Why not simply teach him never to bite? As an adult, say someone steps on the dog and hurts or startles him while he’s sleeping or chewing a bone. His natural instinct will be to bite – hard. But if he’s learned never to hurt people, he’s likely to respond instead with just a growl, a snap, or a very gentle warning bite, rather than a damaging one.
So, to inhibit force: Play with your puppy, paying careful attention to his bites. When the bites don’t hurt, praise him. But, whenever a bite does hurt, freeze, then say, “Ow! Stop it, you worm!” (or something disapproving like that) and take a two or three second timeout. The fun ceases, forcing the puppy to briefly stop and focus before playing resumes. The puppy learns that soft bites are ok, hard bites are not wanted and end the play session.
To inhibit the frequency: Once your puppy is only mouthing you gently in play, start to pretend that soft bites hurt too, even if they don’t. When he’s mouthing gently, praise him: “Good dog, that’s very gentle.” But when it gets a little harder, say, “Ow! That really hurt me!” The puppy learns, These humans are soooo sensitive – I’ve got to be very careful when mouthing this guy.
Eventually, if you’d like, stop the play session if your puppy bites or mouths you at all.
To even better control his mouthing behavior, teach him the cue off and practice this exercise: Let the puppy mouth you, and then tell him, “Off.” When he releases, say “Good boy!” and give him a reward. Then, let him mouth again. To ensure the dog always maintains a soft mouth, continue these exercises into adulthood. Also teach him the rules of tug.
And here's a free download that will teach you just about everything you need to know about puppies.
Good luck and please keep us posted.
"Education is a progressive discovery of our own ignorance." -Will Durant