Giving Access To Another User

Hello, I work for an attorney, and he built his own website using WordPress. He has a blog on the website that he wants me to have access to, so I can post on it. How would we go about doing this? He does not want me to have access to alter the code or anything like that on the website. Only access to post on our blog, nothing more nothing less. Thanks!

The page I need help with: [log in to see the link]