First set a time limit for the child for playing games. Explain the child that this games are becoming too important for him and he shouldn't do so and concentrate on studies more. Talk to him about other things that are more fun than just playing games. Try to make the child involve in outdoor and indoor games and make him more interested in hobbies like singing painting dancing etc. Tell him to read for 3 hours first and then you allow him to play games. And after some days the addiction will start decreasing.