You say he has people interested.  How "interested"?  Does he have an agent who is trying to sell his idea or is he trying to sell his idea himself?  If all he has is a mock up with celeb photos, I have to wonder.  Potential publishers don't want to see some mock up with a bunch of celeb photos.  They want to see concept pieces depicting the characters as they will be in the story.  

Another point not really addressed.  The 3D assets.  I know you have a lot of assets.  But unless I miss my guess, you purchased them for your own personal use. with your own money.  I know you stated he would purchase any new assets needed but that doesn't address all the rest.  Our agency has an extensive library of 3D assests.  We also charge the client a fee for the use of those assets.  If they require an asset not already in our library, they pay the full cost of the asset.  

All I can say is be cautious.  Get everything in writing.  Do not make verbal agreements.  Do not let this person talk you into doing work on the promise that when it's published you'll get paid.  And before you sign anything, get a lawyer to go over it.