How to give developer my cpanel access?

First off, why does he need access to your database outside of installing a theme? It sounds like you've already got that installed. And beyond that, there is little to no reason why anyone would need to connect to the database and look at all the information stored there. Almost all of the information stored in the databased is parsed and visible in some form through Wordpress itself.

Next, you mention he needs access to the files of your site. Rather than giving him a cPanel access, why not just create an FTP login that allows for only the viewing of the files on your subdomain?

To do this, go into your cPanel, then hit the "FTP Accounts" tab under "FTP". Create a login username, password, then under directory you'll want it to look something like this:

The italicized portions there will obviously correspond to your site. If you give him that login information, your developer will have access to all the files available on that subdomain. Including all the files in your Wordpress install. If Wordpress is already installed, you should be good to go! If it's not setup, just give him your username and password for your database (that would be controlled from phpMyAdmin). The beauty here is that because phpMyAdmin is run from inside of your cPanel, even if he's got the login to your database, he can't actually access it with the control of phpMyAdmin. However, the logins will give him the information needed to plug into the wp-config.html file that Wordpress needs to be installed.

Let me explain why this is wrong for future reference. You have an overall login for your cPanel... think of it as a master password. Your databases have a seperate login. It appears you created this through MySQLDatabase Wizard. Giving him this login would allow him to connect to the database if he were say Wordpress needing to read/write to it. But he's not. That login you're giving him is not the master password, and as such, it won't work to login to your cPanel.

So like I said above, I would recommend not giving him cPanel access, but rather FTP access to only the files on your subdomain. No real developer will need table access to your database unless it's horribly corrupted. The login you gave him was for an individual database. He can use this information if needed to let Wordpress establish a connection to said database (again, this is done through the wp-config.html file used when Wordpress is initially setup).

Please let me know if this helps.

Cheers,
Ben