Well, your dismissive attitude probably isn't helping much. :/

"Why doesn't he just copy us and show no fear" - because in his mind, there is danger there, reason to be afraid? It's called an irrational fear for a reason - it doesn't seem to have much bearing on the reality of the situation. Also, phobias don't catch, so it's unlikely his siblings would start copying his behavior.

I honestly wonder if animal phobias come more from secondhand experience than actual experience. I've noticed this trend where people who are attacked by dogs seems less likely to be afraid of them than someone who witnessed an attack or was almost attacked.

I had a phobia of dogs since very early childhood - part of it still lingers, particularly if the dog is unleashed, or aggressive. Exposure and thought seemed to work best for me. Maybe see if you can get help from therapy dog groups, or maybe even a local kennel that will let your son play with a puppy for a bit, then move up to interacting with larger dogs. Basically he needs to learn that most dogs are not threatening.