One of my cats is like this, but not quite so extreme. But she is really flighty. If someone other than me or my partner comes in the house, even if its our friends or relatives who come over often, Sushi (the cat) bolts down the hallway and hides under our bed for about 3 hours.
If there is a sudden loud noise, either inside or outside, she literally flips out (you know how cats jump and do that flippy thing

) and again, runs down the hallway and hides under our bed.
I've had her since she was 5
days old (I was a vet nurse and hand-raised her) so I know that she has never been abused or anything like that. She's just naturally nervous.
I think some cats are just like that man. It's like humans, some of us are just naturally really anxious and hate surprises and loud noises etc. Some animals are the same. If he hasn't even been abused or anything, then it just sounds like he's naturally a nervous cat. Have you owned them since they were kittens?
If it's not causing him, your other cat or you any significant problems, then it's probably not worth worrying about.
However if he is
really distressed when a loud noise occurs, e.g. if he hypersalivates and/or pants (heavy-breathing with his tongue out), or if he goes off his food for a couple of days at a time etc, this indicates he is
seriously anxious and you need to do something about it. In which case, see your vet
It's called Feliway (unless they have a different product in the US?)
It's extremely effective, but very expensive. Therefore you should only bother with it if your cat's anxiety is causing MAJOR problems in the household, or if its quality of life is significantly compromised by its anxiety....or if you just have the spare money lying around to pay for the diffuser and refills
*edit* Oh I just saw that you found the answer already :D
Carry on...
NT, your whole post is completely awesome and spot-on
It's a good point you make about the skinny cats being the flighty ones and the fatties being more laid-back.
Again, think of humans. A lot of the anxious/nervous people we know tend to be skinny, and those of us who are on the larger side are often more happy-go-lucky/relaxed.
Animals are the same.
Hell, we ARE animals so that makes sense right! :D