8 month old GSD barks at males during walk?

[JVJ08]

He is a very sweet dogs and I started socializing him since he was 10 weeks he lets everyone pet him and he is really friendly with other dogs especially the little ones, he's even friendly with cats but for some reason he sometimes starts barking at random males during our walks, his ears point up, tail Stiffens and just stares down the guy while barking at him non stop. He doesn't lunge at em he just gets in front of me and barks at them. what can I do to solve this problem? I am a male and he doesn't bark at any of my friends he just does this evey once in a while