Socialization -- or, more specifically, the LACK of socialization -- is probably the single biggest cause of aggressive behavior in dogs. Lack of socialization causes fear responses in older puppies and dogs, leading to shyness, yappiness, and downright aggressive behavior.
With puppies, socialization is fairly easy... just take your puppy with you, everywhere you go. Anywhere you're allowed to take him... take him. Introduce him to everybody. Make special trips to the park, or the pet store, or walk him in the grocery store parking lot. Show him off. Make sure he understands that ALL people, animals, places, are GOOD people, animals, places. If you notice that certain types of people, animals, or places cause even the slightest fearful reaction, take him more often to be introduced into those situations -- and reward him for appropriate behavior.
Socializing the older dog to strange people, surroundings and strange dogs is a longer process than socializing a puppy to the same people, surroundings and dogs. It's not impossible, but does require more patience. His natural inborn instinct is to fear new things -- in the wild, new things are dangerous. You have to teach him to trust in your judgment.
If you've been given the special challenge of socializing an older dog to strange people, it may not be as difficult as it sounds. There are rules to follow -- try to bear in mind how YOU would feel. Use common sense -- don't allow him to become overwhelmed. Watch his body language -- this will tell all. If he approaches strangers with his head slightly down, tail wagging, this is GOOD -- it's submissive, but happy. Don't allow people to pat the top of his head -- this is an aggressive gesture. Have them offer their hand for sniffing, then rub the chest area -- a natural calming spot in all dogs. Carrying extra-special treats (reserve the really good stuff for use when meeting strangers) will really speed the process If he hangs back from meeting people, don't encroach on his "safety zone" -- allow him to approach in his own way, in his own time. Food treats will help. Reward for appropriate behavior -- appropriate behavior is interest only -- no barking or growling. If the dog barks (not in a friendly way) or growls, you've forced him past his limit.
On the subject of growling: growling at humans is inappropriate. In situations where the dog is fearful, though, it's a first-line-of-defense against the scary things, and the last thing you want to have happen is for someone to teach this dog not to growl. If you find a human that your dog will growl at, the best way to handle it is to teach the dog that this human means good things for him -- really good treats and profuse praise offered in conjunction with the presence of this person will help enormously.
Set the dog up with "chance" meetings. Prepare the "stranger" ahead of time. Have them carry bits of hotdog or cheese in their pockets. Have them maintain a distance that's comfortable for the dog. (This process will work especially well, if you can arrange for several "strangers" over a period of time -- perhaps taking a walk, and meeting 1 or 2 such "strangers" each time.) While talking to the dog in a normal tone, have the stranger lob the treats to the dog, with the dog in a sitting position. Over time, the distance can be closed -- allow the dog to determine how close is "close enough". Never encroach on his safety zone -- you'll be lessening his trust in you, and you may be held liable for the consequences.
The purpose in all of this, is to teach the dog that strange people mean GOOD THINGS for him. This system of reinforcement for appropriate behavior can be used to socialize the dog to any fearful situation -- strange dogs or places, being groomed, going to the Vet's -- anything. Always carry treats , because EVERY situation is an opportunity for learning. Keep the safety zone intact, keep a firm grasp on your patience, and remember: he WILL learn.
Special considerations regarding socialization with giant breed dogs and puppies:
At seven months, your giant breed puppy may very well weigh seventy pounds or more. Do you socialize him with pups in the same age group, or adult dogs in the same weight group?
Age is a much more important consideration than size -- if your pup weighs seventy pounds, and you choose to socialize him to adult dogs because of this, he can be ostracized by the group, based on immaturity versus size. Dogs have no concept of breed differences: those seventy pound adult dogs may look at your 70-pound pup, and deem him socially unacceptable. He won't be taught doggie rules of etiquette -- he'll be excluded from the group, or even repeatedly attacked by the others, which can lead to fear aggression toward other dogs.