There can be many causes for this; try to keep an eye on him and see if there is a pattern of when or where he becomes so nervous or anxious. We had a friend who had come over once when our dog was a puppy and had told him off for jumping up at his young children. It wasn't a big telling off by any stretch of the imagination but it was highly rare that anyone besides us told him off so from then on he would cower away from that person whenever he came over. Dogs have memories imprinted on them of what people do or what may have happened in a place, which can have negative effects on them for years.
As another example, that same dog got to know where the vets was, not helped by the fact that we never walked around that corner with him except to go to the vets, and started walking extremely slowly like he was walking through treacle as soon as he saw where we were going. This would progress to stopping dead at the bottom of the steps to the door. This only started after he was micro-chipped, which he did not like at all.
