annie
I don't know about RuneScape in particular, but World of Warcraft is similar (online, interacting with other players) and my nearly-30-year-old husband often has a hard time making appropriate choices when it comes to the game (as in, he will stay up too late playing, or play during the day when he could-should be doing other things, or he will play for long periods of time until his eyes are glazing over and he realizes he hasn't eaten or been to the bathroom in a good 8 hours...). If it's difficult for an otherwise-responsible adult to manage online role playing games, it would only be more difficult for a child.
I would set strict parameters - only X minutes per day, only after all homework and chores are done, etc. I would also make sure that he continues to get enough exercise by playing outside and whatnot. It's too easy to become a "game potato." Maybe his friends could come over and they could make cardboard swords or whatever and they could play "RuneScape" in the back yard. I'll bet if you get them a few refrigerator boxes to make a fort out of, and snacks, they'd have fun. See what you can learn about the plot of the game, and try to re-create it in an active, face-to-face interaction rather than via the computer, way.
I think if I had a son interested in video games, I would limit him to only playing it on rainy days! I wish DH would listen!! :)
Amy