Montessori, for example, has a more free-flowing structure to the child's day, where they can do an activity when they feel like it, stop when they feel like it. The idea is that then they're always in the right mental state (i.e. engagement) for the task they're doing then.
If the only reason the child is painting, for example, is "because it's 2 o'clock", then the child likely doesn't feel like doing that then.
have your doctor talk to him about it