Lots of flat work especially in the canter and improving their canter so it is bouncy and balanced.
Gymnastics. Staggered bounces (high low high low) but be careful that this doesn't make the horse rush and go easy on them as they are hard on their hocks. Also, low wide oxers can really help to improve their bascule over fences.
I'd say Dressage work. If he's balanced and collected on the flat, he will also be so over fences. Of course it won't happen overnight, it's a long progressive road.