EFL teaching methods for 7 year old with ADHD

I am tutoring one-to-one private English lessons for a Chilean boy with ADHD. The lessons are designed to enforce the material that he has from school. The lessons are in his house. He takes his medication in the morning, but then by the afternoon when I have an hour with him the effects have basically worn off. When he used to take some medication in the afternoon his attention span was clearly much better, his behaviour is now much more erratic; before he used to complete tasks either in order to beat the clock or to win time for games, but now those incentives are not working.
Can anyone suggest some techniques to try to get him to concentrate to the same level as when the medication is working?

Get a small plastic 35mm film canister (or something like that if you no longer know anyone who uses that film. A very small baby food container, perhaps). Then use small dried beans to fill the container whenever the student is on task appropriately. When full, some reward is earned. Or when half full, etc, depending on how successful the student is in that area. This works, because it is easy to do, it is very concrete, and it is very incremental. The student can be helped to keep a record of success, and can compete with him/herself to fill the container more quickly, etc.

PS Anytime you have more than one student, you can encourage them to compete against one another, as well as to work together to earn something...

Anita learntoreadnow