And sorry I got on late

These things are so much easier in person when I don't have to spend the time just getting the run-down.
Lets see: Nervous or anxiety
- Fear, confusion, survival instinct.
- Boredom or lack of mental stimulation
- Illness or discomfort
- Unique individual behavior
- Training and rehabilitation are possible if patient and knowledgeable.
My first guess would be probably a learned/experience behavior.
- What has he been his constant all this time (hen mates, your dog, the coop, etc.)
- Why is he afraid? What is there? What has he been exposed to?
- Why doesn't his flock react the same? Is it a personality thing or a personal experience thing?
- Any past *traumatic experience (like the time the dog jumped into the brooder, etc.)
Maybe these questions can help you narrow the margin, as curiosity is our best tool in this case, keep the questions coming;-) Perhaps (if you can find the trigger assuming there is one) you might use counterconditioning or desensitization to curb the nervousness.
Will try to find time to check this thread during work if providence allows
take care
_twilia;-)