I would try and talk to him about what he would like to be doing in say 2 years time, and then 5 years time (and if that goes well - 10 years time !).
With the starting point that he needs to be financially supporting himself, and hopefully doing something he enjoys.
Ask him if he can see ways that gaming will support him in the future, or if it will just be one of the things he chooses to do in his leisure time. If it will be his income, then get him to think about what he needs to do to get to that stage. What does he need to get out of school to get to where he wants to be ?
If it isn't going to support him, then what else could he imagine doing (or if not that then what would he definitely NOT want to be doing ? Not just work but life aspirations - eg working towards a career, living in a bedsit, owning a car, travelling, working casual minimum wage jobs). Then same question - What does he need to get out of school to get to where he wants to be ?
At this age they need to start taking responsibility and ownership of their life. You aren't dictating (apart from no longer financially supporting) - you are trying to help him see where he wants to go and how to get there.
You can't force them - but hopefully you can help face the future.