I am not an attorney and not a CPA. . .
I would say that you would have to go by the date that his divorce became final. In that case, most common law marriages require living together or having a residence together for a number of years before you are considered "married" I would say based on what you said that you have not met that criteria, but you could still check with your state legislation.
Also, if your boyfriend provided over 50% of your support and you lived in his residence, he could still claim you as a dependant as long as you don't claim yourself. I would check our that option.