First, are you building the car or do you have a shop lined up already that he is aware of? You didn't discuss shopping around for a builder or getting quotes did you? Sounds like you're pretty much all set with arrangements.
Hmmm.....I really couldn't tell you why he so boldly sent them to other people. Obviously, he was aware that you could see the names. On face value, that seems unprofessional. As a professionally contracted artist/business, you HAVE TO respect the wishes of your client. It's an unwritten law.
Now, there may be a good explanation for it and you should demand it from him. You said he does work with a couple of those builders, does he have an email account at their shop so that he can check email there? Not that he should be storing outside work on their harddrive. I'm just thinking he may be sending it to himself, I dunno. Maybe he's sending it to a buddy that works there....not a good idea. All it takes is for him to forward it to others.
Bottom line is, confront him about it. Communication throughout the design process is an absolute MUST. If there is no good reason for him to have done such a seemingly irresponsible thing you may be able to take further action. However, you may or may not have documented and signed any contract of confidentiality with him. It's a good idea from a legal standpoint because there is no question about ethics. There is a difinitive line not to be crossed. If a civil action is necessary, hopefully not, you may have some difficulty proving your case.
I personally, don't take this stuff too seriously but I am perfectly willing and in full support of a client that may want to keep the project confidential. I'll sign and respect anything and everything they say within reason. So far, I'm extremely lucky to have worked with top notch, classy people that I could have a beer with. If I don't think the project is a good fit as far as personality or taste, I respectfully pass on the project and suggest an alternative artist that may work better. In the end, this is a fun hobby for all of us. I never take this stuff personally because it then it becomes "Work" or a "job". And that's no fun!!!
I'm sure some of the other designers/artists will offer more. I really hope this came out right, no time to proof read...Good luck, let us know how it goes.