Have a chiropractor come out- he may be stiff and physically unable to bend comfortably.
Hey! This could be fixed by just flatting him. You can exaggerate the inside and outside bent while riding him, do lots of circles, and lock the outside rein while riding so that he has no space to drift his head the wrong direction. Make sure your inside leg is always pushing him out as well.
My horse used to do this a lot usually I would either put a two poles on the ground like a tunnel for them to go into or to up on the sides of the jump or you could also put one side of the jump higher.
use inside leg it puts pressure on that side and the horse will either move over or straighten out it works wonders on my horse
Maybe your horse is in pain ? His back might hurt him and that is why he would put his head like this... Have you seen an osteopath ?