I'd say landing on soft ground (your enemy) would normally reduce the damage you take. However, no matter how you plan it, right after your sword hits your enemy, you are going to hit your sword. Additionally, the default falling damage assumes you are instinctively trying to roll with the fall or otherwise cushion your breakable bits, in an act of self-preservation. Holding a sword in a position to hit someone is at odds with such self-preservation reactions.
http://www.d20srd.org/srd/environment.htm#Falling Objects
You can be treated as a falling object for purposes of damage done to the enemy. Almost all PC races weigh in at the 101-200 lb category, which means 1d6 per 20' you fall. (He takes 1d6 per 10' you fall if you weight 201-400 lb).
Your own falling damage is treated as 1d6 per 10' fallen, reduced by 1d6 because you are landing on a soft surface. If he is wearing heavy armour, or is otherwise pretty solid (stone golems etc), you would take standard falling damage.
To reflect the fact that you are intentionally not trying to roll with the fall at all, I would roll an additional 1d6 each time a six is rolled.
As for the weapon attack itself, I would treat it as a normal attack, not a charge. The point about a charge is that it is controlled movement, and you are able to adjust your position in mid-stride. Not enough to change target, but certainly enough to correct your aim. Falling is uncontrolled movement, which does not allow you to correct your aim at all. So, no charge bonus.
I'd also want a Tumble check (DC 15 seems to be a reasonable default) to avoid striking yourself with your own weapon. You are after all, quite literally, falling on your own sword, which is a metaphor for something I'm sure.
If you were falling on him with an intent to grapple or overrun your enemy, I might reasonably consider a +4 circumstance bonus on it.
In any case, you end your turn prone in your enemy's space. I would also allow for a free overrun attack to see if your enemy was knocked off his feet.