Has he had a negative experience with another dog? Was he not well socialised as a puppy? These could be reasons why your dog is scared of other dogs.
You need to socialise him more to help him to not be so scared. If you have a friend/relative/neighbour that has a dog, it would be a good idea to get them together to show your dog that meeting other dogs can be a positive experience.
You might benefit from an obedience class, where your dog will be meeting other dogs on a regular basis.