If he's asking for user name and password for FTP it would seem he's got an FTP client, so no need to give him File Manager access. There really is no way to limit access for the main cPanel user so he can be shared, nor to create a sub user with limited access.
What you could do though if you've got an extra domain name available, create an addon domain on top of your main domain and move your files to that directory (which would be located inside your public_html/addon.com/) and then give him access to that addon domain FTP login details. When he logs into that addon domain, he's stuck in there with no way to get to the rest of your file system.
There's a video clip briefly showing how to do this here:
Addon Domains - cPanel Video Production
Or if you like, you could create a new sub.domain of your main domain, but not from your account's cPanel, instead, create the sub.domain.net from your WHM create new account tools.
Of course you might like to first create a special package and feature list just for this purpose and give this to that sub.domain when you create it.
Doing this gives him access to the files you place in that directory, same as above (which would be located inside your public_html/addon.com/), and gives him full access to "a" cPanel, but not your, cPanel. This way he has access to File Manager if you want.
A few options for you. HTH!