Ah Fai, the Dumb(1997)

Mentally-challenged Fai (Eric Kot Manfai) lives a simple life with his mother, but he is eventually abandoned by her when she leaves him to get remarried. Living with a family friend, Fai bumps his head in a serious accident, and the subsquent surgery turns him into a genius. However, Fai's gain in intelligence also means the loss of the innocence he once had.