It sounds like he might be jumping early - at least, that's what the description of what the horse is doing made me think of.
If that's the case my suggestions are:
- I would try setting a pole on the ground about one stride ahead of the jump - that should be his takeoff point.
- make sure that you are not cueing him to jump too early by going into 2 point to soon - wait until his feet leave the ground, pretty much pushing you up and out of the saddle.
- try doing some more trot jumps
* I'm often reading and posting from mobile and Siri loves to make a mockery of the English language.