Your 5yo lacks the mental development (and physical brain development) to formulate the intent to deceive the way you think of it.
5yos don't have voice command software installed, so what they've been told is very often the last thing they're likely to remember in any given situation. It's why 5yos don't get to live on their own yet. They need supervision, not 'rules to remember.' The flea spray should be out of reach, preferrably locked into a cupboard with everything else that should be out of reach (medicine, nail polish, cleaners, poisons), with the key unavailable to him.
5yos, faced with angry (dangerous) parents will say anything they think the parents want to hear, or whatever they hope will make their parents love them again. This make the 'truth' a rather slippery item --because they do not undertand cause and effect yet, or even that the ball will fall to the ground every time (yes, even if it doesn't feel like it this time, even if you wish very hard, even if Santa comes).
This is just an unsupervised 5yo doing the kinds of things a 5yo would think of doing, supervised or not. That he is telling the story afterward in the way he wants reality to have been is normal, and is no indication of moral slippage or inherent evil. The less said the better.
Put the flea spray away.