For people like that it comes down to mentality. People can do whatever they want if they trully WANT it. People want to progress in a job and work hard and grind? People want to save up and grab a 1k iPhone they been saving for??
It's the same with eating right and also working out. For me alot of people fall into the I like the idea of something but dont really want it THAT bad. For me I love food. I can eat all day everyday. I know gluttony would be the 1 of the 7 sins I'd probably fall under.
However I adapted the mentality of reward vs excessive. Eat and workout well enough during a week and then having 2, 3 maybe 4 bad meals for reward. Don't completely cold turkey a habit but adjust accordingly. I got tired of feeling slow and out of shape that it pushed me to force change the habit of eating bad excessively. Some people need coaching and others can figure it out themselves
I would ask you on a scale of 1 to 10 how impulsive is he with other habits? Spending, drinking, etc. Normally theres. A habitual relationship in between someones choices that make it harder to break vs making the change. For me my gym life effects into everything else. When I workout like scheduled, I do better in work. I'm motivated in other areas of my life. I'm more aggressive in taking down goals beyond the one habit of weakness.