Your dog is still a puppy, especially considering that it is a GSD (slow-to-mature breed). I don't really think you have much to worry about; if the dog's bitework is OK (and hopefully you are only working the dog in prey at this age) and he gets along with people, then his nerves are probably fine. Dogs are neophobic by nature, i.e. they tend to instinctively distrust things they are unfamiliar with. One thing I might suggest is to bring your dog up close to a parked truck and let him investigate the truck. Ditto for the cow--except make sure he's muzzled and approach the cow from the front (or better yet, have the cow behind a fence). Dogs do tend to lose their distrust of strange things once they become familiar with them. Hope this helps.
One other thing I've heard is important is your attitude to the dog when this happens. Don't sooth him or give him any indication that his fear is appropriate--but, of course, don't punish him! Instead, give him hearty assurance, sort of like you would when playing with him. Not praise, just an indication that there's nothing to worry about. I find that helps with my GSD, who is indeed a little neophobic, but also quite resilient, especially when he understands that _I_ don't view whatever it is as threatening.
All joking aside, both Pete and Dave hit the nail on the head. Let the dog investigate things he is afraid of, and don't try to reasure the dog by praising. The dog will think you are praising him for being scared. Just act confident and your dog will pick up on this.
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