Scylla is right... if you hope to become a PR through your partner, he needs to make this right before he lands.
If you don't, then your partner can land (although he is still at risk of a charge of misrepresentation), but if he lands without informing CIC of the change in marital status (from single to common-law), then he won't be able to sponsor you later.
Suggestions on moving forward -
You should have filed form RC65 with the CRA two years ago when you became common-law (after living together for one year). You can file this now retroactively to the correct date (it is possible there are some tax implications, but not likely to be significant if you don't have children). Complete this form, keep a copy/scan (you will need to submit this to CIC) before sending it to the CRA.
http://www.cra-arc.gc.ca/bnfts/mrtl/menu-eng.html
Your partner needs to contact CIC, explain that he didn't understand about common-law status, he thought he couldn't include you in the application because you aren't yet married (include info on the planned wedding). He understands now that he made a mistake, and ask CIC for guidance on how to correct his error. To add you to the current application, he will have to return the COPR and wait for your application to be processed before he can land.
Alternatively, seek the advice of an immigration lawyer.