That's a pretty delicate situation. Clearly you care for your brother and for your family too, but talking sense into him is the best thing you can do at this moment. Since I don't know how dynamic are things between you and him, I wouldn't suggest using force to kick his addiction as I can't foretell his reaction. Also, overlooking his first purchases of 500 bucks was a bad idea. Although unintentionally, his behavior got reinforced.
And there could be a whole different aspect to it. He is in his teens. Addiction to gaming is expected. And so are troubles in school. It could also be possible that he is avoiding school for reasons not yet known to you. Teen troubles, bullies, girls. Could be anything. I would watch out for indications. Depression, reclusion, keeping to himself, short-temperedness, ignoring his friends. Do his friends come over to meet him or check him what is he up to? If they are, you can ask them about the school. If nobody comes-- now that spells trouble....