My kids have always left Singapore on their Singapore Passports and entered the US on their US passports. Same on the reverse, leave the US on US passport and enter Singapore on Singapore passport. In your case, he would have to leave on his Canadian Passport. As far as entry in the US is concerned, if you are just transiting, makes no real difference, but if you are stay a couple of days, then he should come in on his US passport and leave using the same. Entry into Canada is via Canadian passport.
SOME PEOPLE TRY TO TURN BACK THEIR ODOMETERS. NOT ME. I WANT PEOPLE TO KNOW WHY I LOOK THIS WAY. I'VE TRAVELED A LONG WAY, AND SOME OF THE ROADS WEREN'T PAVED. ~ Will Rogers