I think that u can divided the study time into small parts. You can ask him to study 1 hour and rest 15 mins like that. You can also give him courage. Ex: if him have high marks like 80 marks, u can give him something as a gift. You SHOULD not always beat him and say he has 學習障礙 and say he is stupid. He will be disapointed when you scold him.
Hope my comments can help you.