I understand your concern about finding the ideal weight for your horse. A body condition score chart will help you determine whether your horse is still overweight. This chart was developed for horse owners to quantify the body condition of their horses. Look at the descriptions for each body condition score and then compare it to your horse to determine where he lies in the range. This is an assessment of body fat so it works for any horse, even if the horse is big-boned or muscular. Take a good look at the actual amount of body fat covering his rump, ribs, neck, and other body parts mentioned on the chart. Avoid focusing exclusively on his belly.

Muscle is hard, fat is soft. In order to differentiate between the two, use your hands to feel places where horses normally deposit fat, like over the ribs, at the tail head, on the neck, and around the shoulder. Performance horses should have a body condition score between 5 and 6. It is particularly important they maintain appropriate weight so they are not carrying around extra pounds, putting more stress on limbs and making it more difficult to cool out after exercise. 

You are currently doing what it takes to get him to the ideal weight. If you are seeing a result, it must be helping. Easy keepers are a challenge, because as you cut calories their bodies become more efficient at using the calories that are available. This is why it is so important to keep the horse in a regular exercise program, which will prevent this from happening. Once the horse is at an appropriate weight, you will still have to monitor constantly, making subtle adjustments in the diet in response to weight gains and losses. 

Because he is an easy keeper, he probably will put on weight effortlessly and will do well with a diet of mostly forage and a balancer-type concentrate. Since a horse can derive significant calories from forage, the amount of forage can also be adjusted, not just the concentrate. Caution should be taken to keep an adequate amount of forage in the diet to keep the digestive tract functioning properly. In general, a horse should not be fed less than 1.5% of its body weight in forage per day. For this 1600-pound horse, that means no less than 24 pounds of forage.

Please let me know if you have any further questions. Perhaps with the body condition score chart in hand, you can show your colleagues how to assess body fat as well.