First - congratulations to your son for three excellent choices. No bad choices here, just different flavors of college.
Size really comes down to class size, so that is something to think about. I would say that, if those Canadian programs are appealing to your son, that they deserve serious consideration.
Unless you guys are extremely wealthy, paying half price is always helpful, even if you can afford the extra money.
Canadian universities are more like European ones in that a lot more of the grade depends on exams, rather than weekly assignments. Some students prefer to make a serious push a few times a year, and reduce their effort in between these, while other students prefer to have a more constant, lower level amount of work.
I did my undergraduate in Israel, where, for many courses, there is only one midterm, and most of the grade is in the final. Many students preferred that, because it allowed them to establish their own schedule for studying. Other students really did not like it, since it meant that a lot was riding on that exam (though every final was given twice, with the last grade being the one that was entered on the transcript).
What sort of rhythm does your son prefer?
Something else to consider is that “college experience”. Canadian universities have a lot less of the American “college life” culture. Again, some students really like it, others think that it’s a waste of time (my kid likes it, but some of her friends at other colleges really prefer having more of their life off campus).
Maybe your son should make a table on which he will have rows for everything he would like in a college, and then seeing which of these places tick which rows, and which rows are the most important.
Good luck!