Lots of transitions and poles on the ground. I would say a whip or spurs but some people aren't a fan of artificial aids.
If you're not a fan of artificial aids, try lottttsssbof transitions. Walk to trot, trot to walk, walk to halt, halt to trot, etc. After becoming efficient with these, you could move onto walk-canter, canter to walk, trot to canter, etc. It'll help wake up the horse, and get his attention. You could also help him physically warm up with exercises like serpentines, leg yields, S-turns, etc.
If your horse is ok with whips/crops try a schooling whip. If not try spurs. I know these options are not the best but its the only way 👍🏼
I would try using spurs to add more impulsion to your flatwork. You might need to experiment which type works best with your horse, but spurs have helped me in many cases for flatwork via getting my horse more attentative.