The vowel sound in "boy" is neither long nor short. It is a diphthong consisting of an "o" followed by an "ee" (long "e") sound. A word with a short "o" sound is "Bob", and a word with a long "o" sound is "bone".
The word 'sneaker' does not have the short e sound. The combination EA makes the long E sound as in sneak and peek. The EA pair does not always make the long E sound. For example EA in the word 'bread' is a short E sound, and in the word 'break' it has a long A sound.