My DS is at home (just me and him). He pays rent proportional to his earnings. Luckily we both agreed on what the appropriate sum was. He does his own washing, shopping, cooking etc, shares other chores. I personally think that the most harmonious way to do it, is treating it like a house share. I don't think it does any good to continue to "parent" them once in their 20's/30's etc. I need to know that if I dropped dead tomorrow, that he could cope!
It took a little while to make the transition of him understanding that if he did something around the house, he wasn't doing it "for me"! But I just reminded him it was for us, and he soon got it.
We have a combination of factors at play, we live in London, he has ASD and while he has lived independently (he worked abroad) he wasn't that keen on it. I'm not sure he was really ready at that point. He doesn't want a relationship either. Not sure if that's down to the ASD or he would have felt that way anyway. So he might always be here. We get on very well, the occasional bicker but never a full blown argument.
It works for us. He gets cheap rent, I benefit from not having to pay all the bills by myself. If he wasn't here I would need to get a lodger so it's better that it's family.
With regards to the gaming, my DS is also into that. I think ultimately it's up to him what he does in his free time. My DS has made loads of friends through it though and they have gaming meet ups, or sometimes all go out for dinner, that kind of thing, so he is socialising too. Maybe you could encourage your DS to think about doing something like that. There's actually a lot going on in "the gaming community". My DS has travelled all over the country to events, even to America once.