Grid work!! It makes them jump better, and their brains/bodies work quicker. I did it all winter and my mare is jumping so much better!
Grid work is the best way. Start low and easy and work your way up. 101 Jumping Exercises is a great book that gives lots of ideas! Practical Horseman also does some grid stuff
Work on exercises that will make him tuck up his knees and work on placing his feet. Use lots of ground poles in front of jumps and work on elevated trot poles and such things. Also, most horses have an awkward phase when it comes to jumping and with practice he will likely work out of it and will learn to see a distance on his own.