I cannot emphasize enough how important socialization is for puppies. In addition to the socialization that they should have gotten from the breeder (see Part 1), from the minute you get them home they should be seeing at least 10 new people a day. The best way to do this is to take them everywhere you go. Many employers these days will allow dogs at the office, but at the very least they might be williing to allow puppy to come in over the lunch hour to meet everyone especially if you tell them how important it is to your puppy's wellbeing.
Walk them past strip malls, schools, bus stops, wherever there are people and encourage people to pet them. The one rule though is that if the puppy starts to jump up, people stand up straight, turn around and say "off" and attempt to pet puppy again when 4 paws are on the floor.
Drop by the vet's office just to say hello, to be petted and cuddled by the vet techs or other clients there (keep them off the floor if they have not completed their series of shots) and then leave again without them having had any type of appointment, shots etc. Ask your vet first of course, if it is ok to drop by for a quick visit to say hi and socialize for a few minutes, most vets will be happy to allow this. Take you puppy to an LRT station and go up the escalator (hold your puppy, don't let them stand on the escalator), meet people getting their tickets or transferring to buses.
Anywhere you can take your dog safely and legally, do it. Expose them to as many sights, sounds and people as you can. Make the visits and exposures positive and if puppy shows fear, tries to hide, barks, hackles, tucks his tail, pees etc, don't punish him or coddle him, just go along your way treating him for being calm and really treating him for showing any sign of interest. Maybe you need to back up a little from the "scary" thing or person in order for puppy to become calm, fine then that's exactly what you should do and reward for the calm behaviour.
Let puppy see people from all walks of life, countries, loud people, quiet people, children, people in wheel chairs, using canes and walkers, with large coats, fuzzy hats, sunglasses as often and a positively as possible and as quickly as possible, like the day you get them home and don't stop doing this ever. Ok maybe they don't need to stop into the vet's office a couple of times a week to say hi anymore, but don't tuck them away at home as an adult, never to see another person outside the family again. Socialization is ongoing, but imperative for puppies.
For socialization with dogs, I recommend puppy classes, a reputable doggy daycare or friends with well socialized, adult dogs with up to date shots and no aggression issues and never leave the dog and puppy unsupervised, which of course you aren't doing because if puppy is not in your direct supervision, he is in his x-pen or crate.
Dogs who are either not socialized as puppies or not socialized enough as puppies with a wide scope of experiences tend to become fearful and unfortunately often show this fear by way of aggression against the scary or unfamiliar object, person or dog can end up causing physical injury to people, dogs, property and themselves, possibly causing fear aggression in other dogs and can in some cases even redirect their aggression on their owner.
You could end up sued for damages if the dog bites, fined for the bite by the loca municipality, shunned by your neighbours, your dog could be forced by the municipality to wear a muzzle, or worse euthanized for the behaviour.
Unsocialized dogs, even if they don't ever actually bite, are often surrendered by their families to shelters and rescue groups. They are difficult to rehabilitate and it's very hard to rehome them. They are often euthanized.
So let's nip this one in the bud and socialize as much, as often, as in as many places, circumstances, different people and as early in puppy's life as possible.