WordNet (r) 3.0 (2006):

sonny
    n 1: a male child (a familiar term of address to a boy) [syn:
         cub, lad, laddie, sonny, sonny boy]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "sonny":
   boy, bub, bubba, buck, bud, buddy, child, colt, cub, daughter,
   descendant, fellow, fledgling, foster child, grandchild,
   granddaughter, grandson, heiress, hobbledehoy, lad, laddie,
   manchild, master, muchacho, offspring, pup, puppy, schoolboy,
   scion, son, son and heir, sonny boy, stepchild, stepdaughter,
   stepson, whelp, young man, youth