Answer:
Metaphor
What is a metaphor?
A figure of speech in which a term or phrase is applied to something to which it is not literally applicable in order to suggest a resemblance, as in "A mighty fortress is our God."
In the sentence, "My son is a greyhound out of the blocks when it's hometime," the son is being compared to this because of how fast he runs to get home.
Hence, the technique used in the phrase, "My son is a greyhound out of the blocks when it's hometime" is a metaphor.