Hi
You have to list your son as a non-accompanying dependent if you don't want to sponsor him at this time. Your partner is not required to sponsor him. If you want to sponsor him later when you get your PR, you can do that as long as he's still studying because while he's still studying, he's still dependent.
For outland, you send your application to Mississauga and once they are done with it, they will send it to London.
Why complicate matters, especially if the son drops out or takes a gap year. Include him as dependent in the application, as accompanying, he gets his PR as although he is over 22 he has been studying since before 22. He gets his PR, comes to Canada, "lands" returns to his studies. He can then remain outside Canada for 3 years before his PR is lost.
PMM