While your grandson might not be on board with the idea over a fancy gaming chair, I'd personally favour a good ergonomic mesh office chair. Especially if any time is spent at the computer during hot weather, those big leather gamer chairs cease to be comfortable pretty quickly in the summer assuming they even were to begin with, as well as being worse for your back/posture in the long term.
I know that Noble gets a good rep as far as gamer chairs go, but I'd still opt for a quality refurb from the likes of Herman Miller or Steelcase every time. That said, there's some pretty damn good Chinese knock offs for a lot less, Sihoo comes to mind and I think Andaseat (usually more known for gaming chairs) has started doing similar in the form of the X-Air but know very little about the range.