Looking for a fantasy book.

Hello,

Im looking for a boon that I only vaguely remember the ending of.

The main character was male and at the ending was offered to go back in time to the moment of the start of the book. I remember that he requested ink and put it in his mouth so that the ink would make his teeth black and make him remember the events that happened before losing his memory and travelling back in time.

Some similarities to When You Reach Me; see if it looks familiar.