Christopher doesn’t tell lies, not because he’s a good person, as his mother said, but simply because he can’t. He has fond memories of his mother, who smelled good and wore a fleece jacket.
Christopher implies that his dedication to truth is not his own choice, and so it says nothing about his morality. His mention of his mother makes it clear that she is no longer in his life, though he still does not explain why.
Christopher can’t tell lies because only one thing ever happens in a certain time and place, and there are an infinite number of events that didn’t happen then. If Christopher thinks about one thing that didn’t happen, he begins to think about all the possible—and often absurd—things that didn’t happen. Thinking about these possibilities makes him overwhelmed and frightened. He doesn’t like normal novels because they’re not about real events, and so everything he’s writing is true.
Christopher’s dedication to truth is revealed to be in fact a need for truth. Truth keeps his mind on track and keeps him safe from the overwhelming stimuli of the outside world and potential imaginings within his own brain. The fact that lies and fictions cause him actual mental discomfort provides a greater motivation to tell and seek the truth, both of which he does throughout the novel.