If he hasn't been playing regularly with other dogs, and is more than four months of age, you have a bit of a problem, as he is past the optimal socialization period. I would try to find a good positive trainer who holds group classes, and has a play period at the beginning of class and sign up for the lessons. If you are in the United States, you can check IPDTA, APDT, peaceablepaws.com to find a trainer in your area.
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Fergie, Retired Lap Dog, Age 20 1/2
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"If you talk to the animals they will talk with you and you will know each other. If you do not talk to them you will not know them and what you do not know, you will fear. What one fears, one destroys." - Chief Dan George
"The fidelity of a dog is a precious gift demanding no less binding moral responsibilities than the friendship of a human being. The bond with a true dog is as lasting as the ties of this earth can ever be." ~ Konrad Lorenz