Go look in my FAQ (something like "how do I import a car from Canada"), and you'll see a link to the NHTSA (or was it customs) all about importing a car.
If he stays for less than some minimum time (around 6 months?), then he's just a visitor, and does not have to comply with any laws.
If he stays for more than that minimum, then he has to import the car, and meet the safety and the emission laws.
Don't let him just show up at Kennedy with his car in a jumbo jet and hope for the best. U.S. Customs has rules, and they will be enforced.