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