This depends on what the realisation causes him to do.
1. If the realisation causes him to do something, you could describe the metaphor as:
"The realisation pushed him to act, a spur on his legs"
"The realisation spread through his body turning the keys of his body sending him into a frenzy of motion"
2. If the realisation causes him to have an emotional change, you could describe this as:
"The realisation created a pause, a hidden tension rushing out of his body, water down the tap
"A scream of anguish ripped the air, as a poison, slowly eating away at his heart, released."
3. If the realisation causes him to know something, e.g. a scientific secret, the metaphor could be:
"The realisation hit him, he became Archimedes in his bathtub, for a second."
"The idea spread through his body, over his tongue, and he tasted it, a soft, caramel flavour"