I really doubt Iodine changed his personality; the change is more likely the result of pressure from coworkers plus increasing health problems that made him realize it could get even worse if he doesn't take action.. and now that he feels there's something he can do to address it, he feels motivated to regain his health as soon as possible.
If I were you, I would persuade him to do high intensity interval training. You get to exercise AND save time for other things like movie night. I'd also tell him not to overdo anything. Sometimes we're all tempted to go to extremes in an attempt to feel healthy again. But oftentimes those extremes end up having the opposite effect, further damaging the body instead of healing it. That being said, I personally think it's good that he was able to cut out pizza. I highly doubt it's because he suddenly doesn't like the taste of pizza anymore (although once you start eating other things, I guess your tastes could change); I think it's more of him trying to be dedicated to the goals he's trying to achieve. The only problem I see with him refusing to eat not-so-healthy food is if he binge-eats them a week later.