Just take your pup everywhere with you! If he's had all his shots, he can walk next to you. If he hasn't, just carry him when you're in places frequented by other dogs.
I went out of my way to show my pups different things. You know how with kids mothers will point out very little thing in the sun? "Oh look! What's this? It's a snail!" or "Oh let's go walk on the sand! Do you like sand? Yay sand!" or whatever. Do that with your pup. You will look silly to others, I guarantee you, but it's worth it!
You need to socialize him to diff sounds (loud, soft, high pitched, squeaky, low), sights (everything from parks to construction), people (young, old, skinny, fat, wearing hats, holding umbrellas, with beards), animals (other dogs, pups and adults, cats, rabbits), flooring (sand, gravel, grass), etc. But ensure each interaction is a positive one (don't push too much too fast - work your way up from a quiet park to a shopping mall on weekends, and I praise and treat for interacting with strange things) and be careful never to compromise your pup's health and safety.