I agree with whomever talked about the movie, supersize me. Sit down and have a movie night, and pop it in!
I had to watch my 3 nephews and niece often, and I'm still amazed at what their mother lets them eat. but that's not my concern, but if I was to watch them for a week or longer, then I felt that having a go at their nutrition was fair.
GIVE HIM A CHOICE. not, "what do you want to eat?" choice. I don't know if it's possible for you to make dinner, and I know it can be hard if you're a full time mother. but if you can, have 2 menu plans in mind. present them to him. "you can either have broiled salmon, or blasalmic chicken, which one do you want?" these are the only 2 options, and you give him power of choice in the situation, but you still hold the control. he won't starve and he won't eat fatty or unhealthy food if it doesn't exist in the house.
if he's stubborn and says "neither" then say suit yourself, but if that's the case, I'm going to make so-and-so for myself then. if you get hungry, then feel free to have some. while making the dinner, have a bowl of veggies and dip out so if he happens to wander in the kitchen in search of something to eat, then there's something right there, and he doesn't have to put any effort into making it himself, so he will probably nibble on it.
so, get rid of all junk food in the house, or other unhealthy options. (mac and cheese isn't necessarily "junk food" but it's a quick and unhealthy meal for an 11 yr old to make for himself)
since you are a full time mother, and i'm unsure of your schedule, Make sure you stock up on food that's convieniant AND healthy. there shouldn't be substitutions. it is possible to get nutritious and delicious foods that are easy or quick to make. when you go shopping, whenever you pick something up off of the shelf, ask yourself, is this convieniant? is it healthy? don't get food solely on the convieniance factor.
After your son kicks the "addiction" from substances put in burgerking and subway foods, he will realize that he wants to eat healthy. eating healthy will become preferred, he will have more energy and more enthusiasm for life.