It really doesn't matter. Who's last name you choose, makes absolutely zero difference in your application.
If you are applying for PR as Type A dependent of your parents, then in addition to not being married you can also not become common-law with your bf. So if you live in the same home with him for 1 year, that would also cancel your dependent status on your parents application.
Since you have a child together and will have a long relationship by the time you land as SINGLE, you should be prepared that CIC may want to do an investigation to make sure that you were not really common-law when you landed.