Grandson with dog phobia

My DGS has always had his little quirks but at 9 some of these are becoming more pronounced. His phobia about dogs particularly when they bark is making life quite difficult. He is very reluctant to go for a walk in case he meets/ hears a dog. We live in a village and he knows where the loud dogs are. I had plotted a new route for school avoiding the house with the really aggressive dogs which I think started all this but we had to turn back before this morning before reaching school. Have any other grandparents experienced this and have any advice to offer?