Ease him in slowly

For starters, you should always try to bring your dog out for regular walks. This will really get him acquainted with the surroundings, which will be foreign to him. From the footpaths to the high rise buildings and even to random animals, your dog will need to understand that there are vast things in the world for him to explore. You should really vary the people that your dog meets too, so that your dog is exposed to the idea that you are not the only person in the world. This will help to prevent your dog from developing poor responses to new people such as fear or aggression when the puppy is older.
One thing to note in this process is that you should keep your puppy on a short leash. This will really allow you to have time to react when things are not going as well as you hope.

Don’t over react

I can attest to this.
My dog was on a leash at a park nearby, and she went around sniffing another dog that happened to be in the area. After some sniffing, the other dog began to growl and bark, while my dog stayed still and seem confused. The other owner pulled her dog back and subsequently accused me of not controlling my dog. This is a classic case of over reacting to a simple event. When this happens, the other dog will not understand the meaning of socialization.
You should always stay calm whenever such situation occurs. If you over react when the dog whines or barks, it is a signal to the dog that these people are dangerous, while the message that you wish to convey is the opposite. Always stay calm and reward your dog when he shows the right reaction to the situation that he finds himself in.

Take your dog to the dog park

Check out your local listings for dog parks around your area. It is perhaps the best place for your dog to socialize with dogs of all breeds. Be mindful of your puppy’s safety though, as there can be unfamiliar situations and dog may need additional protection from hurting themselves, such as near creeks or other running water. But generally, it is really safe for your puppy to mingle and play with other dogs while at the dog park. Of course, having said this, take note that you should ensure your dog is fully vaccinated so that they are immune to diseases that might be present in other dogs such as Parvovirus  and distemper.
A handy piece of advice is to bring along poop bags so that you can dispose your dog’s waste properly if he decides to defecate in the park itself or on the way home. Having a few treats on hand will also help to provide a quick reward for a well behaved dog.