Letting him float won't help. He can still re-up just as soon as if you killed the chute. All he has to do is end flight. He'll get either the bailed successfully or capture just like he would if he floated all the way down.
Did you get the proxie kill? You should have.
I'd save my ammo unless he's close to either a friendly or enemy base. I've stood at the end of an enemy runway shooting guys in the head with my .45 as they take off causing pilot wounds. Likewise I've hidden in the maproom killing loads of troops with my .45 and scoring proxie kills as fighters auger around me.
[EDIT] You can also kill the manned ack at a VB with a .45