Posthumous authorship or acknowledgement
There may be laws that govern this, though laws vary from place to place. Normally he would have to give permission and that &quot;right&quot; may legally have passed to his heirs. You would do well to contact someone responsible for his estate and take up the question with them.
But since he never gave permission, you don't have permission, yet, to include him. You have an obligation to acknowledge his contribution and probably cite some of the specifics as well, even though they haven't been published.
Dedicating the book to his memory would be a nice touch also.