I forgot to mention that this will help keep him interested on you
Lots of changing directions (3 loop serpentines ect) transitions, hacking, dont have a set routine and just do all different things every session
In my opinion it is good to do a short sessions but make him put more effort. So instead of having him walk for a longer period of time practicing exercise, it may be better to do more transitions, stands and keep changing directions.
Keep your horses mind focused on you. Keep reminding your horse so your legs and hands. If your doing a circle, go through the middle and change rein and keep doing different things. LOTS OF of walk to trot and trot to walk transitions the keep his mind occupied. If that doesn't work do lots of small jumps and poles
poles poles poles they are probably the best things I have discovered for a bored even nervous horse . very easy