Question Author
Thanks all for your prompt replu and all good advice. I think I give my dog plenty of socialization. I took my dog to puppy classes ever since he was 12 weeks old, and had first several shots. He then went to many classes. He has graduated from advance class, and received AKC Canine Good Citizen certificate. I took him to puppy social 2 -3 times a week until he is old enough to go to doggy park, now he goes to doggy park daily for about an hour and half. (I also walk him an hour each morning). He plays a little bit with other dogs, more like to play with people in the park though. It started with one tall man we encountered during morning walk, he was pretty friendly, and my pup didn't seem to have any problem with him at the first place, but when he start use high pitched voice baby-talked to my pup, bent down try to pet him, somehow my pup getting irritated, I could see his fur on his neck stood up, and he started growling. The man back off. I ordered my pup sit again, and appologized to the man. After that incident , He seems start stay alert when a tall man approach him. Including some people he used to play with. He has no problem with family members, or any regular height man. And I noticed, many tall man in the dog park, they let my dog approach them first, then pet him, my dog accepts them fine, and often goes back to them ask for more screch. I think I will try let the man he fears feed him method next time. Hopefully he will soon get over this.