Would a 45 year old man ever get something like anorexia?

It sounds like your brother has become a health nut, but it does not sound like anorexia nervosa, self-starving by not eating healthfully. Your brother's focus is healthy eating. 

A health scare can precipitate big changes, like quitting smoking, drinking, red meats and processed food, as your bother has. This will cause a big weight loss until the lost unhealthy calories by are replaced by healthy calories, which will be a challenge because the food and drink he quit are so calorically dense. Give it time and see what happens. Chances are that he will gradually increase the calories he eats until his habits have changed to provided him with the nutrients he needs to sustain a healthy weight. 

Also, bear in mind that you are accustomed to seeing your brother as a corpulent man. If you check BMI charts, you'll find that the range of healthy weights is rather low in relation to what we have come to see as normal in America. Is his weight within the healthy range, or is he underweight? For instance, a six-foot man is not considered underweight unless he is below 140 pounds. If your brother is six foot and below 140, I'd be concerned too, but his doctor would probably be OK with it. 

Let your brother know about your concerns and ask him to tell you exactly what he weighs and check it against the BMI charts. If he is truly underweight, write back and we'll talk about how to proceed with an intervention if needed. If he is still a healthy weight, leave it alone and see what happens in the future.