How do I socialize my dog?

I recently adopted a 1-1/2 year old puggle who has obviously never been properly socialized. He has lived with 2 other dogs from 6 weeks old until 5 months ago when he moved in with an older couple (we adopted him just 2 weeks ago). They seldom had visitors and took him for 2 10min. walks a day (the rest of the time he was in the backyard). He is terrified of EVERYTHING but my problem is his aggression toward other dogs, especially those larger than he is. What can I do to introduce them without worry?