That could be a million different reasons.
If it only happens to this one person its almost certainly not a problem with the forum. He should try cleaning out his cache, restart his internet connection, test a few other sites, try pinging your site. If he gets replies from your site name when he pings them it would probably be a problem with his browser and not his connection. can he try viewing the site in another browser.
404 is a page not found, are you sure he is viewing the correct url?