It’s likely that your puppy has not been adequately socialized with other dogs. Alternatively, he may have had a bad experience with another dog. The best way of overcoming this problem is to take him along to puppy socialization classes at least once a week. This will help him to integrate with other young dogs around the same age as him, and learn the intricacies of pack code in a safe envionment. As he grows older you can progress to training classes, again on a regular basis to help you learn how to train him and for him to learn how to interact sociably with other dogs.