I am not a parent, but I have first-hand experience of being addicted to video games and getting through it as a teenager.
I. Video game addiction is a very real thing. Whether your child is already addicted or not, though, the following advice applies if you want to reduce his time in front of the computer.
II. The real cause of the addiction (or just playing for too much) is not the game itself, although some games are more addictive and hence more harmful than others. The game is a place where your child attempts to hide from the world outside.
III. Therefore, the first thing you have to do is treating your son's problems that lead to playing too much, not the symptoms (actually playing games). Get a therapist for your son. There are several very important caveats here:
- It's probably not going to be cheap. But the consequences of being addicted will, in the long run, cost you and your son a lot more than good therapy, and the need for treatment will still persist.
- It's probably not going to solve the problem instantly, or even to show any significant improvements instantly. This problem didn't build up for just one month, and it won't go away in one week.
Your son might not think that he has a problem, and not want to cooperate. You need to have a calm, peaceful conversation with him about this issue: ask the counsellor you find for details, since I am not a professional here. Anyway, before your son realises that something is wrong, you are probably not going to go far from where you currently are.
The counsellor might give you and/or your son some guidelines, but most part of the work will be done by you and your son, not the specialist you hire for help. You will need to follow those guidelines if you wish to overcome your problems.
IV. Find alternatives. There are many fun and useful things that a teenager can do instead of playing computer games. For example, when I started attending fencing classes at the age of thirteen, where I could swing a real sword against other real people, doing this twice a week made playing most computer games just boring. Likewise, laser tag immerses you into action a lot deeper than any shooter game can. Switching to tabletop role-playing games like D&D will at least make your son go outside and communicate with real people instead of characters from his games. Countless things: tennis, football, historical reenactment, horse riding, photography, hiking, cycling, skiing...
Even if something like this doesn't completely remove your son's interest in computer games, the amount of time they spend gaming is likely to drop.
The alternatives you find and present should not be forced upon your child. Instead, suggest them and let him make the final decision.
V. If all or most of his social activities happen online, he will definitely need new social contacts. Especially if they are all related to video games.
VI. His problems at school might be a consequence of many more factors than just playing for too long. My grades didn't improve after I had stopped playing for 18 hours a day. Instead of studying some "boring and useless" stuff, I would usually do something else, be it spending time with a friend, reading a book or watching a movie.
However, a good counsellor should be able to treat those problems too.
Also, no, studying is not going to become his alternative to gaming, because games are designed to be fun, and studying, sadly, is not, though it could be.
VII. Educational computer games are usually very boring, not fun, and hence won't help you. And, when he sees that it doesn't help, he will be a little bit less likely to cooperate with you.
VII. Neither will your problems be solved by accusing and shaming him. Out of guilt, he might stop playing for, say a week... and then start hiding the fact that he started playing computer games again. You probably don't need this, aye?
VIII. Treat your son as an adult, not as a child. Fifteen years is a point where many methods of parenting either don't work, or do more harm than good. It's a hard age: your son often behaves like a child, you treat him like a child, it sounds logical to you. But what he needs is learning to behave like an adult, and when the whole world treats you like a child and tells you that you are a child, there is no way to learn.
For example, the first temptation might be to just restrict video games. You could install parent control programs, just forbid him from playing, or even remove his computer, perhaps along with any other multimedia devices.
This won't help, though. Your son will find ways to cheat and play stealthily. He will bypass your parent control program, find a friend to come to to play computer, play from school's, use his smartphone or another device to play, etc. What is more, the problems that lead him to playing games will stay. And he will need to overcome them -- albeit later, perhaps after ruining his career and social life as an actual adult, rather than now, as a teenager, when mistakes don't cost much.
Your son is younger than you are, and he is a lot more kin on technical stuff. He grew with it, and the odds are, you didn't. Even if you win this competition, the very fact of it happening will damage your relationship and make it hard for you to help him overcome his addiction. Or to help him at all, with anything. You have more life experience, but, unless you are an IT specialist (which you could be, considering that you came to Stack Exchange), he is most likely smarter than you when it comes to computers.