OMG!! Soooo many people responding to a Runescape question! LMAO!
Listen. I am a 31 y/o guy. I play Runescape. I love RS! I and a couple of my buddies play online together several nights a week. It is addicting, and i know of some ppl who play it up to 8 hours per day or more. 60-90 minutes per day should be plenty of time for your son to do what he needs to do in the game.
Runescape has many more pro's than cons. It teaches kids about long term goals... you can play this game for years and not ever do everything there is to do. It teaches them how to have patience. There are a lot of things in this game that are boring to do, but the rewards for doing them are worth the work. It teaches them how to follow rules, and how to get along with others.
About sexual predators: it does not matter what your kids do online, they are open to this kind of deception. If you want to truely protect your son from sexual predators, lock him in a closet.
Runescape is kind of intense. It is mainly because of all the work and effort you put into your character. You don't want to see all your hard work get wasted. After all when you have worked for something (even in a game) it gives a sense of satisfaction, and the threat of its loss WILL upset you.
My advice is to sit down and talk to your son about it. try to let him understand that you respect his RS time, but at the same time you think it might be unhealthy for him to be spending 3, 4, or 5 hours a day playing it (since it is).
Below is a link to the OFFICIAL Runescape Parents Guide, and a wikipedia article about Runescape, of interest to you would be the part at the bottom about a study done by Brunel University about RS.