Not all dogs roll over and play submissive puppy when attacked assertively. He felt he needed to defend himself and frankly if your description is accurate then I don't blame him one bit. However, this does mean that you have to be extra diligent about your socialization of this pup. Male Mals can get cranky - so now that he has been scared you need to do 'double time' on helping him feel safe. This scenario possibly undermined his trust in you. In his mind you let a strange man attack him. So now he might feel the need to defend himself first before letting something like that happen again and he might start defending you too because he doesn't think you can 'read' people quickly enough to make a safe decision.
I am not saying you did anything wrong - but its all about whose in charge of keeping the pack safe in his doggy brain. And this incident did nothing to build his confidence in your leadership. It might have even shown him that snapping works to keep the world away and he 'might' try it again if he is nervous enough. I am not trying to scare you but you need to heighten your awareness for the future. Take charge, read his body language and be ready to correct him for a bad choice and reward a good choice.
This man was out of line and totally at fault.
Love Them & Lead Them,
~Elizabeth & Doug
Dog Training the Way Nature Intended