Your best option here would be to stick with his existing account.
While you can create a new one, if you do so, you are truly starting fresh. Under his account, you'd have access to all the books that had been previously purchased. With a new account, you'd have to buy any prior content you wanted again.
The easiest route here is to just reset the password ***** his account so that you can register your kindle to it, then you can update the credit card info as well.
There are a few easy steps to fix this though.
First we need to reset your password *****: http://amzn.to/1gxYWzC
Then update your card info:
First, go here:
there you can be sure your card/bank info is correct (right address, valid expiration date, etc)
From there, go here:
On this page, click on the SETTINGS tab at the top right,
then you can from there set a default card to be used for one click purchases on the kindle.
Under Kindle Payment Settings, click on the Edit Payment Method button, and you can set the card you want used for your kindle purchases.
After that, reboot the kindle by pressing/sliding and holding the power button for 20+ seconds, and it should operate normally from there.