At around 6 months old, most dogs reach an awkward adolescent stage (although Large breeds this can occur at 14-24 months). Everything you taught your dog seems to be forgotten, he won't come when you call or even think of obeying the commands "sit" or "down".
During this time the young dog is attempting to assert his rank, just to see whether he can get to be the boss. You will probably need to do a lot of obedience training. Praise is very important and even an older dog should be congratulated when they do something right.
Don't allow him to get away with too much. Suppress even the first signs of aggression with suitable measures as soon as they manifest. This sort of behaviour may be difficult to cure as he grows.
If he acts aggressively against humans, he has not profited from important training measures. For example, you should only feed your dog when you have already eaten and you should avoid giving him anything from your own plate. When your dog gets his food before you eat, he will think he is the boss.
Try taking your puppy's bowl away some times when he's eating, or encourage him to give up chewing bones or playthings without complaining. If he growls or even wants to bite, react rigorously. Put your dog into "place" or lay him on his side. Hold him down gently with your hand over his neck. Let go only when your puppy surrenders and relaxes.
This dominant soothing technique can be used any time when he is misbehaving and won’t hurt your dog if carried out correctly. Indeed, it is similar to the behaviour a higher ranking dog in the pack would show to put a younger dog in his place. Let the puppy get up only when you think it is right - even if he begins to struggle.
This is also a gesture of dominance among dogs and serves to strengthen your position. It must be clear to all members of the family, that the dog is the last one in the family hierarchy. In order to quell your puppy's first ambitions to climb up the hierarchy, he should be forbidden to sit on the sofa for example, or to sleep in the bed, unless you explicitly allow him to do so.
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