I
-'s contracted form of
▷ is s, z, ɪz —The rules are identical to those for the 3rd singular present ending (except that it is not usually used after a sibilant and that there is no strong-vowelled variant): the boy's asleep ðə ˌbɔɪz ə ˈsliːp
II
-'s contracted form of
▷ has s, z, əz —The rules are identical to those for the 3rd singular present ending: the boy's begun ðə ˌbɔɪz bɪ ˈɡʌn —except that on the rare occasions when it is used after a sibilant the pronunciation is BrE AmE əzː the bus's arrived ðə ˌbʌs əz ə ˈraɪvd