My boyfriend, also my common-law partner, just becomes a PR of Canada. I am holding a 3-year post-graduate open work permit, which will expire exactly 1 year from today. Since I spent last two year working aboard, not in Canada, I am unable to apply for a PR status on my own.
Have you and your boyfriend lived together for one year at least? If not, he can't sponsor you as his common-law partner yet; if yes, he should have declared he had a common-law partner on his own PR application, and since he didn't, he won't be able to sponsor you now.
If you haven't lived together for a year yet, then I suggest you live with him for a year in Canada, and then get him to sponsor you as his common-law partner. Make life easy on yourself and get a lease or mortgage with both your names on it, a joint bank account, utility bills in both names, and other financial proof you are a couple.