Hi Graeme. First off, congrats on the measured response and the relationship you have with your son. It sounds like you're doing something very right, and working pretty hard at it. I like the fact that you offered to block the site in question, and did so with your kid's permission/agreement, and your reasoning. When someone mentions the word addiction, it's a quiet plea for help.
Like most parents who give a damn, you're looking for guidance, some hard-and-fast rule to rely on. I can promise you that if you find one, it'll be dead wrong. You're going to always have to rely on your own judgment, and motives. You know what? So are your kids. If you can impart a sound process, you'll be successful as a parent. We learn the most and best when we don't even know we're learning, though.
As your kids develop and mature, you're going to need to express more trust in them. By the time they're 17, they'll expect more privacy online than they have now. But you already know that.
Trust your gut, and be honest with yourself. If your motives are good, chances are, you'll do ok.
Gotta say, I like this thread.