I agree with the others, a normal reaction from being grabbed by the neck. But with that being said, Goku's hearing is much better than ours, so he must have heard the man approaching and wasn't so much startled because he came from behind a truck, but rather from being grabbed. You want him to be trusted as others said, with kids UNEXPECTEDLY grabbing him, no matter where. I've heard of a Mal recently put down by a Rescue because he had bitten a child when the kid fell on his back while he was asleep. Unfortunately, the dog never got over being afraid of kids and acted agressively towards them afterwards. You also mention he's sensitive around his neck area. I'd also be working on that. You want to be able to grab Goku on any part of his body without being afraid he may not like it and retaliate. I'd love to see pics of him, it's been too long
Also how's his coat doing?
Edited to add: I remember you said you may show him, the judges will feel his testicles, so you may want to test him there too, lol!