A commitment ceremony is not legally binding as a marriage is, so it's not necessary for your partner to be divorced for this ceremony to go ahead. However, the ceremony will not give either of you the same legal standing as husband and wife. As a matter of interest, a person can enter into a de facto relationship while still married to another person.
Just for information's sake, your partner can apply for a divorce as a sole applicant, and he does not need the consent of the other party for the divorce to pass. He will need to attempt to serve the application on his former spouse, but there are exceptions to this where the other party cannot be located.