Just to add to above, see here for more info (should apply to either sponsor or applicant): http://www.cic.gc.ca/english/resources/manuals/op/op02-eng.pdf
5.38. What happens if the common-law partner (principal applicant) is married to another person?
Persons who are married to third parties may be considered common-law partners provided their marriage has broken down and they have lived separate and apart from the spouse for long enough to establish a common-law relationship – at least one year. In this case they must have cohabited in a conjugal relationship with the common-law partner for at least one year. Cohabitation with a common-law partner cannot be considered to have started until a physical separation from the spouse has occurred. A common-law relationship cannot be legally established if one or both parties continue their marital relationships.
Officers must be satisfied that a principal applicant is separated from and no longer cohabits with a legal spouse. This evidence may be in the form of a signed formal declaration that the marriage has ended and that the person has entered into a common-law relationship. An officer may require that the person produce other written evidence of a formal separation or of a breakdown of the marriage. Acceptable documents include...
So basically as part of common-law application you will need to state he is still legally married to someone else. You would then include proofs such as his formal declaration the marriage has ended, and photos/proof of religious marriage (even though it's not "legal" marriage). The fact you have a child together would only strengthen your case as common-law. If you want to apply quickly, there is no need to wait until legal marriage as you should easily be accepted under common-law rules.