depends if he had declared non-resident or not. but by law, if he got any tide (like a house, family to live in canada) then no matter if he is physically in canada or not, he is suppose to pay the income tax.
and i got confused, if you and your son is not a canadian citizen, then he needs to pay overseas student to study in canada, then does not matter if your 66 apply the application here in canada or back in HK. your boy could not enjoy the benefit of being a local student...