The dopamine we ADHD folks get from things that we absolutely LOVE, will unequivocally mean we aren't interested in doing other things. Gaming seems like a hyperfocus for him, and whilst it clearly keeps him happy, he can't sit and game for the rest of his life.
I am a gamer but I don't game every day, and I only do it when I've made sure to do boring things first that need my attention, such as laundry and the like.
He's a teen so he will also be all hormones and rage and possibly have a wobbler if you take it away from him, but unfortunately, if you don't start to reduce it, it will take over absolutely everything.
Not doing social things outside school etc is a symptom of being overwhelmed, if you were to insist he went to a sports club or the like, you should expect to see a meltdown. He will also likely feel exhausted from masking at school all day, so gaming is likely his form of comfort and escapism.
Unfortunately, if you want his gaming time to decrease, you'll have to impose some limits. People might say that this is a bad idea, but those of us with ADHD do respond well to structure IF we are allowed time to do the things we love. People often get angry with us for not being able to self regulate, but this is because our dopamine levels are crap and therefore anything that doesn't give us that rush won't interest us one iota. This is why we get called lazy, but we don't have that switch in our brains that says "hey, we know this is boring but it needs to be done, so do it."
We often don't see how bad things are until they are overwhelming, and then we get "stuck" BECAUSE of that overwhelm, so we can't get through it without help, and then we don't ask for help because of shame, and then the cycle perpetuates until it gets worse.
I would gently suggest that you tell him that there are going to be two nights a week where you do something together as a family that doesn't involve gaming. Often we neuro folks don't realise how fun something is until we are shown. Rather than asking him what he wants to watch or do, organise something and make sure he's there. He might be restless and fidgety, this is normal for ADHD because many of us can't sit still or remain in the same position for long if we are doing something we are bored with. Watching me do things in a kitchen is hilarious, rather than doing something in one place like a normal person, I will end up spreading it across the entire room and then think, oh shit look at this mess. It's eyeroll inducing and embarrassing.
He might have a meltdown if he's not allowed to game, or maybe he will go into freeze mode because we do that too - if we have no interest in something we just....freeze.
If he's just been diagnosed then I'm guessing he hasn't been given any medication yet? If he hasn't he will need to go through titration for that. It takes a bit of time.