IIRC you fall approximately 600 feet in the first round and 2000 feet by the end of the second round.
And falling 600 feet onto a flying horse's back isn't going to do any less damage than falling 600 feet onto the ground, except that you'll hurt your flying horse, too.
A better question might be, can a falling paladin dismiss his mount?
No. Paladin mounts are no longer PokeMounts. They aren't extraplanar creatures summoned for a brief time by the Paladin. They're wholly of this world and can't be unsummoned or dismissed. They can be summoned to the Paladin, but that's only so that if the Paladin leaves his horse at the front door and exits the dungeon through the back door, his class feature doesn't get lost for a month.
A better question might be, can a falling paladin dismiss his mount?
No. Paladin mounts are no longer PokeMounts. They aren't extraplanar creatures summoned for a brief time by the Paladin. They're wholly of this world and can't be unsummoned or dismissed. They can be summoned to the Paladin, but that's only so that if the Paladin leaves his horse at the front door and exits the dungeon through the back door, his class feature doesn't get lost for a month.