What is significant about this quote is that it is given a whole chapter to itself. No more detail or description accompanies this one sentence chapter. This clearly makes this mention of Pi's mother's birthday stand out and emphasises the sadness and the sense of loss and grief that he is clearly feeling. The fact that he knows his mother has died and yet still tries to commemorate her life by singing happy birthday to her is a moment that expresses his grief and pain at her death.
Of course, at the same time, the need to commemorate her birthday also shows his need of companionship and his desire for somebody to share his experience with. When we read this chapter, we are reminded once more of Pi's youth and his massive solitude. There are sometimes when even sharing a lifeboat with a Bengal Tiger is not enough for the human spirit, and Pi's remembrance of his mother's birthday helps to commemorate her life and also highlight the horrors that he faces now.