You are a fine friend for being concerned about the long-term health of your friend.
Unfortunately, there isn't a lot you can do to make changes in his diet -- it's his life after all. You can cook healthy meals in your flat for the both of you from time to time, but otherwise he has the right to choose his diet for himself.
By the same token, you also have a life to live. You can hope that your good eating habits and yummy meals will entice him to broaden his choices of food, but he may not change. As much as you care about him, your primary purpose in life is to live your own to the best of your ability. My suggestion is to enjoy him for the qualities that made him your best friend in the first place, and learn to accept that there are things about him that you don't have control over.
Here are some cyber-hugs for you, because I know that it hurts to see our friends like this --