My 8 Y/0 Grandson Bite His Nails Until Their All

my 8 y/0 grandson bite his nails until their all gone, now he bits the skin off around the tips of his fingers. he is afraid of the dark and being alone. he has difficulty falling asleep and staying asleep. He craves only sweets does not eat vegetables and fruits on a regular basis. He is constipated often and has poor concentration along with below reading grade level. Parents divorced two years ago.