I collected his book before speaking with him or his father's ashes.
Now I got no possibility to give him the book, since there is no dialogue option?

What now?

As far as I know, there's no fix for this, because

*Minor Spoilers*

it looks like it's planned this way if you already learned the language from the Ashes. The only thing you can do is rolling back to an older save, if you have one. Anyway, this shouldn't be game breaking, as the quest would just help you learn the language. So if you don't mind the XPs you can go on without giving it back.

Quinn, from Quinn's Fixpack commented on this issue a while ago:
"I wanted to fix it at one point, but can't really do so without adding dialogue which I don't do in my Fixpack unless it's -clearly- a bug. In this case, yeah, you can't return the book if you learned from the Ashes already. It seems designed that way. It rather sucks, as you pretty much need metagaming knowledge to get the maximum xp out of the episode, but there's no simple fix for the matter."

I had the same problem. Use the dodecahedron. You won't get all the way through, but opening it without reading knowledge gives new dialog options with Finam. You can then get through the rest.