With the amount of grass your horse has you may be over feeding him. You said he is on lush grass. Add that to 2 lbs. Of food and 1 to flakes of hay each feeding, that's too much for the amount of exercise he gets. I agree to get him on a NSC or metabolic feed if anything at all. The grass and hay is actually plenty then supplment to make sure he is getting all of the vitamins and minerals he needs. You can also feed him loose minerals...(A handful a day is fine) You will begin to see the weight come off. The older guys have special needs. Even if we think 20 isn't old anymore!