Generally, gymnastics and grids can help for almost anything. I would probably want to use ascending oxers with V poles up for a horse who tended to be lazy with his shoulder or who didn't have a good way of using himself, and put up placing poles to get the horse to jump from the base of the fence. Bounces are handy for this too, but any gymnastic with quite short distances will encourage the horse to sit down, round over his back and lift the shoulder rather than hurdling.
It's also important to make sure that you aren't blocking the front end by pulling back or getting too far ahead of him in your position and actually stopping the horse from using his shoulder effectively, and gymnastics are obviously a good way to help develop rider position. Your only job once you're in the gymnastic is to stay straight and maintain forward motion. Other than that, let the horse do his job.
And then, of course, it needs to come from the flatwork too. If he doesn't use his shoulder on the flat, it won't suddenly start when he jumps. Often these horses simply aren't sitting enough on their hocks to rotate the shoulder quickly enough. Raised trotting poles, transitions, spirals, counter flexion and counter canter - all the usual stuff.