It is common for good doers to be fed less than the recommend amounts of a low energy mix or cube which keeps the energy (calories) down but also means your pony isn’t getting the right levels of vitamins and minerals. The first thing I would suggest is changing the cubes for a low calorie balancer such as Dengie Hi-Fi Balancer. Try this for a few weeks as you may find that just increasing the intake of vitamins and minerals will help.
As he is in regular work and isn’t overweight, it is also possible to try adding a few oats to the balancer. Oats contain starch which is a source of quick release energy and so may help to give him a bit more sparkle. You can add a very small amount each day and then increase a little bit the day before a competition to see if it gives him a bit more sparkle when you need it. It is very important to use as little as possible to avoid him putting on weight and by using the balancer you can use as much or as little of the oats as you need to give him the energy you want. If you try them and they have no effect, remove them from the ration as they increase the risk of other problems. We would not recommend using oats if your pony has had laminitis.