Hehee, this might be hard to take. :)
No matter what anyone says, you cannot 'spot reduce'. Don't believe me? You can do sit-ups till you're blue in the face, but you'll never see your ab muscles if you don't get rid of the fat first.
Fat comes off together. When a horse works out, it will lose fat from everywhere... It may not tone all of the muscles everywhere, but fat will not be lost in just one area. If your horse had a big tummy and he seems to have lost it, it was probably a hay belly (hay and some grass sit in the intestines and bulk it up... after steady exercise, gut movement is improved and the hay doesn't sit in the intestines for as long).
But! With that being said, there are moves to tone up the shoulders, so that when the fat does come off with cardio-work, you can see it :)
Counter bending is a awesome way to work up shoulder muscles, as well as making your horse move out at the trot (which I saw sonny doesn't, heehe!). Counter bend at the walk and trot, and then move to the canter (also an excellent, excellent way to get control over a horses shoulders and get him working off the hind end at the canter! Yay
) Don't forget basic cavelleti (sp? Hm.) either!
Does that help?