Here's the official word from the Celtics media relations team:
To our knowledge right now, Paul remains the sole captain of the team.
As for the web page in question, Brian says:
It's actually a typo, the only two pages on our site that we do not control are the roster and stats pages which are controlled by the NBA….we have alerted them to the typo and hopefully it will be corrected soon.
While that sucks for Ray, I am thrilled that the Celtics actually answered my email and acknowledged the existence of Red's Army!!!!!