I don't know why but this idea just popped into my head. I would try to incorporate games where you record or video him. You could start by just having the camera in the room and then slowly get the video involved so you can capture him in English on video or audio.
What to do with this?
1) You can turn it into a game and watch yourselves speaking in English
2) You can show his parents he does know more than he is letting on. Maybe if they watch the videos together he will stop being so shy or evasive about speaking in English infront of his parents.
Be careful with this though. I work with adults so I am not sure how kids will react. He might love it...or hate it. Just an idea though.
Hope that helps!
LEARN ENGLISH BY USING IT: ANSWERS & MOTIVATION FOR ANY ENGLISH LEARNER