Cindy's Answer

This is a very normal GSD behavior that we often see happening around 4-6 months old. The key right now is to stop taking him on walks, you don't want him rehearsing this. Puppies bark out of uncertainty and if you continue putting him in the position to be worried and then he barks and the problem goes "away" he's actually going to do this more and more. It's much harder to fix if he's had a lot of practice.