I have a whole lots of domain names with yahoo and it is packed in one email address. I have one website that I am developing through a website developer. I have to give him access inside my yahoo server and I don't want to expose all my data's to him. How can I create another account for him top access the server with compromising my privacy?

Change the password or pass phrase for the time that he is expected to be working on it,after is done, make a new one.

If the company or individual is legit, no worries.

If you do not know them, cannot find them, or have no trust in them, do not waste your time doing business with them.

create a new FTP account in FTP section (if any, depends on your hosting plan). You can also link folder to this account. In that case only that folder will be visible to the user.