How do I socialize my dog?

My dog is a 3 year-old Pit/Mastiff Mix - he was a sight unseen adoption - that is to say the first time I met him was the day he arrived in New England from down South. He's very loving and affectionate with people - he's watches over my young nephews lovingly. He sucks around other dogs...he only really has one dog friend. A Boxer pup who was 6 weeks old when he met my dog. Unleashed dogs have run up on my dog in the park and he has responded. It seems the breed of dog he dislikes most are retrievers.