Addiction is a very good word for it. It's either addiction or a terror of not succeeding pushing the guy to do more. Many people in his situation would have burned out and gotten themselves fired or quit long before. I can't tell you how many people I know who have gotten themselves into an abusive situation (work, relationship, etc) and felt they could not do anything to change it.
Stephen King said that there are people with hobbies. When those hobbies take over their lives, they become compulsions. Some people compulsively collect stamps or teapots or what have you, not very helpful. King's compulsion is writing macabre fiction. He says he's very lucky it pays well since he'd still be doing it even if it didn't. He can't not write. When I hear about people getting lost in World of Warcraft I think "Wow, just imagine if they got into daytrading." As I understand it, daytraders can be profitable but the kind of person who makes money at it tends not to have any other life. They wake up to stock reports, hit the foreign markets before the domestics open, are hitting the foreigns long after the domestics close, brush their teeth while reading the financial papers and go to sleep with CNBC on the tube. Now most people look at that sort of thing and think "My God, that's like a horror story." But to those people, it's an itch that continually needs scratched.