There isn't really a quick way to maintain the weight of your horse. The best thing you can do is monitor it.
Buy a weigh tape and print out a conditioning chart. You may even want to draw up a table (date, weight, neck, barrel, hind, overall).
1. Every week record his weight on a weigh tape or weigh bridge. Next condition the different thirds of the horse. You can find out how to do this on Saracen feeds or world horse welfare. Record the scores. Use this information to produce an overall score (good, medium, poor)
2. Think about the exercise they are getting. If they aren't getting enough up it. If you haven't time look at their feed. Likewise, if they are getting enough. Walk is the best to burn fat as trot & canter, hard, fast or strenuous work burns carbohydrates and ironically encourages fat storage. Exercise is not just about riding. You could split feed into several buckets/haynets to encourage movement in the stable. Living out will also encourage exercise.
3. Think about their feeding programme; is it high in energy? Contain Alfa-Alfa? Contain Oils? If so cut down on these or change them to low energy things such as HiFi lite. Then monitor your horses impulsion during exercise because changes in diet will change temperment and energy levels. Also, cut down on treats and fruit and veg in their feed; perhaps just 1 carrot or half an apple. (DYK: there is as much sugar in one polo mint as in one whole apple). If you use clicker training, feed the smallest amount of food you can get away with. Also remove all molasses/sugar based licks.
4. Look at the grass; is it young? is the growth fast? Is there too much? Are you feeding hay as well? Consider getting a grazing muzzle and/or strip grazing the field. Try to avoid shortening the time a horse goes into the field as research has shown a horse that knows it will be brought in eats more in a shorter amount of time than if left in the field to eat naturally. This, amongst other things, is why living out is much better for a horse. Surprisingly it does not affect the amount they eat drastically. In fact a horse is healthier eating grass and forage in a natural way than with lots of hard feed and hay.
5. Now consider the horse's hay. Use baileys forage calculator to work out the amount your horse needs. While your horse is overweight cut this by 1kg then when it reaches the right weight gradually add it back on. You could soak your hay to strip it of sugars but the remaining water should not be poured down drains. Instead it should be evaporated which is impractical. This will also strip it of nutrients and minerals so you should feed supplements or invest in a redmond rock.
6. A lot of people say that a horse will need more weight on in the winter than in the summer. Nowadays high tech / heavy weight rugs mean that this does not apply. If he is bare in the field or clipped you may need to think about this.
7. Continue to monitor your horse even after he has acheived the correct weight as this will be useful when it comes to worming anyway.
I hope this is helpful and useful. :-)