He can't be allowed to disappear into video games if he is slacking off on school and chores. Pull the plug. If he has ADHD, he may throw a huge tween tantrum. So be it. Tweens and teens need their space, but don't let him disengage from you and life so much that you never get him back.

Get him involved in physical skills classes: shop or carpentry. Take him outside and go hiking and climbing and biking. If desk work isn’t his forte right now, then find him something at which he can be successful. ADHD children, in many cases, simply have a low tolerance for bullshit.