Exercises to stop a drift to the right when jumping

[Hovis_and_SidsMum]

So The Destroyer has a bit of a bad habit of drifting slightly to the right when hes jumping which is not too horrific when hes showjumping as the jumps are wide enough but skinnies xc are totally unforgiving of this little trick. He either ducks out or takes a passing flag with him.
It doesnt matter who's riding he still does it, his back has been checked, saddle checked etc - nothing wrong.
Ive tried poles to channel him but he just jumps those as well so I'm looking for some experienced peoples advice. That would be you guys...
Help?!