I have the same problem with my Chi. When I first brought Bear home, she hated him. She would growl at him & snap at him if he got too close to her space. It's taken a good three weeks, but she is now used to him & plays with him too
Yes, it's true that if they're not socialized by twelve weeks than they will have a harder time of becoming socialized. My suggestion would be to take him to an obedience trainer, or enroll him in obedience classes. Places like PetCo & PetSmart offer them at a reasonable price. Some people on here have said that they didn't like the classes held at both of those stores, but it really does depend on the teacher you get. Ask if you can sit in on one of the classes so that you can watch the instructor & see how her or she interacts with the dogs & their owners. If you don't find a class to your liking, there are many people who will do individual sessions with your dog. Some behavioralists even offer aggression classes to people whose dogs have aggression problems. Talk to your vet & see if she knows of anyone that specializes in training dogs. Good luck!
Amy