Many Canadian medical schools require the MCAT. The DEC programs that @MYOS1634 mentions are only available to Quebec residents and is similar to the combined 6 year BA/MD programs that are offered at many US universities.
The only disadvantage for your son in applying to US medical schools with a Canadian degree is that he would not have instate status for public medical schools that give preference to instate residents. Even if he attends undergrad in the US he would still not have instate status as long as you live in Canada.