If this book is self-published (sounds like it is based on "very indie") it's not likely that he'll sell more then a handful; self-published RPG books are a dime a dozen and the vast majority don't go anywhere except the author's mom's attic. So I wouldn't accept anything like royalties or prints-exclusivity in lieu of cash.

At the same time though, you probably don't want to charge the same rates you'd charge for pro work to a dude doing what's basically a vanity project, because the level of distribution isn't really comparable and he probably can't afford it anyway.

I think the idea of a flat rate for the entire book sounds most fair for both of you.