Just try to involve him slowly in other things. My nephew was the same way from about 17-22. He never did anything but game. He lived indoors. His friends and family called him a vampire cause he never went out in the sun and was up all night.
All the negativity did NOT help.
Eventually we started to get him to come to more outdoor events (BBQs, hiking, etc). At first it was once in a blue moon, but every time we made sure that he had fun and when he wanted to leave and go play games again, we did not stop him.
In time, he came out more and more, and even began to organize some stuff himself. I saw a completely different side of him, was pretty cool.
Now, he still games, but in much much smaller amounts. He loves it, but he loves other stuff as well.
Was he an addict? Probably. But you can't force your choices on someone else. You have to give them positive options and let them make their own decisions.
Worked for us anyway.,
Good luck with your brother OP!