He doesn't own his story as you can only own a "work". He has copyright in his autobiography. You have the copyright in your script, irrespective of what it is about. You have agreed that it will be joint copyright so he can't just change a bit and then say he owns the copyright. It is still joint if that is what the agreement is. If the script is exploited then you would both fall to be paid. However, if there is no written agreement as to joint copyright you might want to take the stance that you own the copyright in the whole thing in that the agreement for it to be joint was never finally agreed. Happy to discuss. I hope that helps. Please leave a positive rating so that I am creditd for my time. Also read part 1 of the Copyrights Designs and Patents Act 1988 which will tell you all about copyright ownership.
He can't claim sole copyright under any circumstances unless he has been given an assignation or licence from you. Happy to have been of help. Don't forget to leave a positive rating so that I am credited for my time.