Either he never got a title, missing documents or something, or he can't get access to the title, in someone else's name or being held by someone.
I bought a truck years ago from out of state and was missing one form the DMV wanted. I couldn't get a response from the seller, so I bought a package of the blank forms. I dug out a typewriter, filled in all the information, and then signed the form. I took it to the DMV with the signed title, Bill of Sale, insurance, and Smog Check form. Got new title and plates with no additional issues.
Unless it's stolen, or you don't have the authority to sell it, there's usually a way to get it done. Some people just give up too easily.