The first step to take to help improve your horses condition and muscle tone would be to add some oil into his diet. This is often the easiest and most economical way to help improve a horses condition, especially for horses like yours with a limited appetite.
This can be done in a number of ways as you can either introduce pure oils such as linseed, vegetable, or soya oil or a manufactured high oil supplement. When feeding a significant amount of oil to the horse it is important to support the diet with the correct nutrients, which is why high oil supplements such as Sumo Original are recommended. This is as Sumo Original will supply the horse with super high levels of oils and fatty acids alongside the necessary nutrients specifically antioxidants, Vitamin E and C along with Selenium and Zinc which will allow the horse to utilise the oil efficiently.
Sumo Original contains 50% oil, but is very low in starch and sugars so if you are finding that you are having to stable your horse more than you would like there would be no reason to worry as it provides a source of slow release energy which is non-heating. This helps make it suitable for horses that tend to get easily excitable, as well as those suffering from laminitis.
As Sumo Original is very calorie dense I would recommend adding it into his current diet very slowly, building up to a rate of 50gm per 100kg bodyweight, which is 250gm per day for a 500kg horse. This will help to ensure an excellent overall condition is achieved, by encouraging weigh gain and improving topline. Once condition is improved you can then reduce it to a level that will maintain condition and bloom.