You don't wait until you're in the air to cue him. I start this training by cantering over a ground pole. It is all about what you are doing on your approach. If you wait until you are "in the air", about all you could do is duck, which is a no-no.It seems to me like that would be kind of hard considering the fact I only jump about 2'3''. There's not much "hang time" to ask for a new lead in the air, so usually the horse lands on the same one it started on and you ask for the change after.
I jump 2'3" also and if I'm asking properly (I'm still getting the hang of asking properly consistently) over the jump, most horses that I ride will pick up the correct lead. There is one horse that we have though, who isn't very bright (sweet as anything though), very stiff (we either call him a 2 by 4 or the freight train) and very off balance so he has problems changing leads over jumps, partly because he's not capable of doing it and partly because he gets confused as to what we're asking. And some of our horses are just really lazy and don't change leads over jumps unless you REALLY focus on it and kick the fire out of them.It seems to me like that would be kind of hard considering the fact I only jump about 2'3''. There's not much "hang time" to ask for a new lead in the air, so usually the horse lands on the same one it started on and you ask for the change after.