The main thing is variety! A mix of hacking, schooling, jumping, loose jumping, lunging and in-hand work. And plenty of days off! Easy goals and finish on a good note no matter how short the session is! If you reach your goal 15mins in then it's ok to stop!
Change what you do with him, try not to be repetitive. Play games on and off his back. Take him out of the arena, if you're able to. Introduce him to new things. Rewards are good too. When you have worked, say you have a goal and once he's reached a step toward that goal, he gets his reward.
To not do everyday the same... Hold it short, before you start, think What you want to do, and if he did it go, just stop for that day. So he gets it, if I do something good I Can stop working.
Play with groundwork, go for a walk