I would just ask him that what he had gained by watching films.This would make him realise that he need more concentration in studies than watching films.I would ask him to set a target and study that will make him active and show less distraction towards movies.
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You can make him understand that watching too many films is not good for him and he should concentrate more in studies.You can make him watch educational channels like discovery ,Nat geo etc.You can tell him how watching so many films are putting a bad impact on him.Like his grandes may continue to be bad, his eyes may pain and if he doesn't play or do any physical activities then he may become unhealthy.He will lose his concentration and couldn't read properly by which his grade will not improve.You can talk about it to his parents and advice them to stop their child from continuing to watch so many films. 
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