A.,
I am a Mother who also takes charge of fears to get my kids through them! I believe in de-sensitization or confronting your fears.
There was a picture of a vampire bat in one of our volumes of the children's encyclopedia. It was bearing its teeth and my little girl would scream and run away from the book every time. I sat her in my lap and held my hands over all but the eyes and said, "see, that's not scary." I would do the same thing for each part of the face "see his little piggy nose?" "See his fuzzy ears?" Once she got de-sensitized to the parts of the picture, I showed her a few together until we got to the whole picture. I also told her that he was laughing really big (instead of screaming like it looked).
OK point is that de-sensitization with dogs is the same way. Get a picture book, or get online and look at different pictures of dogs. Get some really funny-looking ones of chihuauas and some St. Bernards. Talk about the different breeds...Shitzus came from China, Afghan hounds from Afghanistan...etc. Ask him what his favorite one is. Tell him how dogs get to know each other...by sniffing. That they can tell who you are by the special way you smell. (my kids can smell if something came from a neighbor's house and can tell me which neighbor, ask you son if he has any friends who have a special smell.) Laugh about how dogs smell each other's bottoms when they meet, ask how he would like to have to smell someone's bottom when he met them. The more you can get him to laugh and identify with dog-related issues, the better he will get. He can even pretend to be a dog...make some paper ears and make a headband. Paint his face to look like a dog. Get a large picture of a dog and talk about the parts of a dog. Tell your son that when he meets a dog, he has to pretend he is a dog. Teach him to make a fist that looks like a paw and have him show it to the dog so he can get smelled. It may still take a lot of coaxing, but I think it will work for him and he may be a bit excited to meet a dog once he knows how.
NOTE: it is still ok for him to have, and you should encourage, a healthy fear of dogs. Your son should never touch or "meet" a dog without asking the owners first. Tell him that dogs are like people, many are really nice, but some may be grumpy or in a bad mood and not want to be met. My neighbors had a dog that was "really gentle" and had "never bitten anyone." This was not an awful lot of consolation to me as I stood in the bathroom looking at my cheek where the doberman caused a 14-stich hole. I had just been sitting with my friend in their living room watching tv, petting the dog. Anyway, good luck and have fun.