The "Ash" you know is a Ditto that has taken the appearance of the real Ash, a boy who died in a car accident when he was 10. The father was driving and died too. The boy's biggest dream was to become a pokémon master, and was just about to begin his journey with Ditto, his closest friend. Out of grief, the pokemon took the appearance of his master and has refused to let go of it since (This explains why Ash does not age). It also promised to fulfill his master's dream to honor his memory.
The mother is the only person who knows of this but she has begun to treat the ditto as her real son (and may have deluded herself to think he is, in fact, still alive).
So yeah, his parents are probably some random dittos who hang around in the grass somewhere.
