Which supplements can give a sluggish dressage horse more energy?

Hi! The first step is to check that the whole ration meets all of his needs. You can do that using the feed-ration calculator on hastsverige.se (Swedish feed-ration tool). After that you can experiment a little with energy sources. Oats are the energy source shown to affect reactivity, so perhaps feed a smaller amount than you have tried. You could also adjust the work a little and add canter intervals once a week, perhaps outdoors, to boost his motivation.