Or assuming they can't restore his account (unlikely) he'll start grinding again from the beginning and get paranoid about account security, and you've given him a justification to sever his last links with humanity and retreat further into his little virtual world. People in MMOs roll alts continually starting from square one just to have something to do. I deleted my own characters once to "get away" and six months later I was back and spending extra time to work to where I was originally. An MMO is incredibly cheap, WoW runs on almost any computer, costs $15 a month and you can bum wireless off a neighbor. It's hard to cut someone off unless you completely control their life.
Two things have gotten me away from MMOs. The first was getting to the "end" and realizing how artificial it all was. When it really sinks in that you're paying them to work ten hour days hitting buttons like some trained monkey to earn bits of code a developer could gift you with in seconds for the purpose of inflating a DPS number, the magic sorta disappears. Deleting his character might trigger that response but it burns all bridges if it fails. Someone had a much better suggestion about getting him to try a private server where you can give yourself all the stuff you'd normally pour hours in for. That really underlines how artificial it is.
The second thing that got me away is just being away from the computer. He probably doesn't actually enjoy the MMO more than real life, it's just easier than actually going outside. If you can lure him away from the internet for a couple days with a vacation or maybe even manufacture some technical difficulty with the connection, once you've got that initial wedge in he'll probably be a lot more pliable.
Really though you can try a couple things but if he adamantly wants to live through the internet there is not much you can do to stop him. I know plenty of people who pissed away money by failing out of college for all sorts of stupid reasons that had nothing to do with MMOs. If he can pay the bills to support himself it's just an unfortunate choice he's making, but he's an adult. It might hurt your feelings and he might look back on his life and feel it's wasted, but at least it's pretty innocuous as far as "life problems" go. Otherwise let him live in a tent in the backyard with no internet for a couple weeks, that's pretty good at convincing moochers :P