At first he will meow constantly, but after a while in the car he'll want to go to sleep, because it's his time to take a nap.
About his breath, you'd have to consult with a vet. He could have teeth or gum
issues, and he could be feeling pain when eating. If the problem is oral, then he may suffer from Gingivitis, an abscessed tooth, bacteria in the mouth, oral ulceration, or even periodontitis. Now, many of these causes can affect your kitty's mood.