6-7 hours a day is simply too much, and experts agree on that. Your gut feeling is right, and it's not about whether other parents let their kids do it - it's about your son. At that age, he needs physical activity, sleep, and other activities, or he'll run into real problems with concentration, sleep, and later with his mental health too.
The core issue: you're trying to negotiate right now, and that doesn't work with this kind of addictive behavior. With a 14-year-old, you need to set clear boundaries - not discuss, just do it. Something like "PC is off at 9 PM, done" or "maximum 2 hours on school days". Yeah, there'll be fights, but that's part of the process. Important thing: the boundaries have to be really consistent, even if he complains or ignores you.
Try to figure out at the same time why he's getting so into it. Is it social pressure (all his friends play), or is he running away from something else? Depending on that, it might also make sense to look for something else together that he enjoys. But the rule still has to stand - that's not negotiable.