I found a great local daycare that does monthly small-dog "play dates" in a doggie-daycare center, thought
www.meetups.com - try there to see if there are any for puppies in your area.
My dog (3.5lb, 5.5mo. old) has no problem with big dogs, largely because he has no fear of them. When another person or animal is afraid, a dog will sense it an act accordingly. Big dogs are getting aggressive toward your pup because they can tell he's afraid. Best thing to do, as has been mentioned, is to keep socializing him, but start with smaller dogs, especially other puppies, so he can learn to interact without being afraid.