I'm not an expert on the U.S. system and my kids are still young enough that my personal experience is rather dated but I'll take a crack at this anyway.
Application process - my general overall comment is that admissions here are based on marks and that is all (at least for most undergraduate programs/schools). I think Waterloo is an exception to that and it is more complicated, not sure if UofT would fall into that boat as well. There is lots of info on the UofT website but I couldn't access the actual forms to see what you have to fill in. I know it was pretty simple back when I applied to university - picked a program, sent marks and waited for a yes. Admittedly, that was a while ago so hopefully someone with more recent experience can jump in with current info.
College experience - like the U.S. there is a wide variety of schools here so it is hard to generalize the "experience". UofT is a very large urban school. According to this
https://www.macleans.ca/schools/university-of-toronto/ ~10% of students live in residence which means a lot of students live off campus which is going to be different than a smaller university town with lots of students living on campus. I suspect it would be very comparable experience to a large urban U.S. college.
Some other random thoughts -
Cost of living - the U.S. dollar is strong right now which will help and Canadian tuition is pretty cheap even when you have to pay international rates (although that depends on what you're comparing it to - a State school vs MIT - there is quite a range). I'm not sure how scholarships work for international students but many of my coworker's children have received amazing scholarships here. Toronto is not the cheapest place to live in Canada but it is likely cheaper than many large U.S. cities. Flights to Boston aren't insanely priced and driving isn't impossible.
With a UofT comp-sci degree he should have no problem with applying to jobs in the U.S. Many of the big tech companies recruit from there (as well as many other Canadian schools).
I would suggest that he looks into coop or internship programs if possible. I'm not sure what UofT has to offer along those lines, I've worked with some UofT grads but I can't recall coming across any coops or interns (it has been a while since I've been somewhere that hired students at all). As an alternative option, Waterloo Comp-Sci coop would be VERY competitive to get into but the work experience puts grads at the head of the line when they graduate. It is also great for students to figure out what they really want to do before they are looking for full time work (and earn some cash along the way). There are many other Canadian schools that offer coop (generally it extends the B.Sc. by a year and you do 4-6 4 month work terms) or internship (in between 3rd and 4th year you work for 16 months) and different schools offer different programs (it can vary by program - where I went Engineering did internships, comp-sci did coop for example). Application for those programs is often more competitive (again based on marks - or at least it was back in the day). I'm not sure how work visas (or whatever paperwork you'd need) would work exactly - that would be worth asking if he went that route. Most schools have some international placements (including the U.S.) so they should be able to sort all of that out.
Drinking age - the drinking age in Ontario is 19. If you look at other schools, it is 18 in some provinces. I mention this for grade skipped students since it has the potential to be obvious in first or second year (rather than later on if they live where it is 21).
You might want to check this out -
https://www.macleans.ca/education-hub/ Every year the magazine publishes a big university guide and rating (maybe check your local library for a hard copy). There is lots of info there to go through. As with any rating system, there is lots of room for arguing the specifics but it gives some ideas.
