There's probably nothing you can do. You don't say what the project is about, or what you might possibly need support for, so I'm forced to assume that you're going to be running CentOS/RHEL in a common configuration on commercial-grade hardware. And if that's true, then your boss is right.
But more importantly, recognize this: the CIO is your boss. He made a decision, you questioned it, he reaffirmed his position, end of story. You deploy CentOS. If or when you need support for the OS (and not the application you're paying for), the blame will have to come back to him since it was his decision. And unless you weren't smart enough to get it in email, there's a paper trail too.
Now make way for the comments from other bitter Slashdotters who will tell me I'm wrong because they've allowed themselves to be scapegoats for their bosses' inept decisions.