I don't think it will be a problem as many Canadians cross into the USA with their cars every day, and many US citizens cross into Canada with their cars. As long as your son has a valid driver's license and the ownership papers for the car, he should be fine. I would check with my insurance company to insure that he will be covered while driving in the USA.
Since I am no expert on this matter, I believe you should contact the Canadian and US border authorities to verify all of this.