From personal experience, with Ritalin, when it is a little above its ideal dosage, it can cause what I call 'hyperfocus' which makes the brain sort of slow thinking. It becomes hard to task-switch or do complicated things.

This definitely manifests as apathy, mild dysphoria at times, and lack of motivation. Reducing the dosage would be #1, but if he doesn't want to take it.. it is his decision..