You are very wise to get on top of this problem while you are still in the MOM role - teens become even more defiant and have more control over poor-choice meals when they are out.
Everyone in the family needs to get on the same page, and now. You need to make ONE meal for everyone that is healthy. Also, you need to involve everyone in the cooking process - it's fun and it makes meal time take longer than fast or processed foods.
Michelle Obama is working hard on this so it's very much in the media. Jamie Oliver is too.
Be sure you make him healthy lunches and do not let him buy school lunches. Get rid of the junk in your home - don't just deny him access and let everyone else have it. There are healthy alternatives/shifts you can make as others have suggested. Whole grain/whole wheat instead of white flour, sweet potatoes instead of white (roast them in the oven in a little olive oil for healthy "fries"), lower fat meats for burgers along with whole wheat rolls, grilled or roasted vegetables. Hide vegetables in other foods - like pureed butternut squash, skim milk and a smaller amount of low fat cheddar cheese on whole wheat pasta instead of junk mac & cheese, grilled chicken (coated with a tiny bit of canola oil and some herbs), whole grain pancakes with blueberries and lots of wheat germ in them and just a dab of real maple syrup instead of all-corn-syrup junk, and so on. Make lasagna with whole wheat noodles, a mix of cottage cheese and tofu instead of ricotta, and add spinach to the sauce, using tomato puree instead of bottled sauce. It's cheaper and healthier. Make twice and much and freeze the rest to save on cooking time. Make whole wheat enchiladas with beans (fat free refried as a spread, and other whole beans sprinkled in) with cooked chicken, a little corn, lots of peas, and some plain tomato sauce thickened with tomato paste, and jazz it up with any herbs/spices you want like cilantro or chili powder. Serve with brown rice. Make Chinese stir-fry with low fat chicken or beef, and LOTS of vegetables with out a sugary high-cal sauce - I use a little soy sauce and some chicken or beef broth. Veggies can include water chestnuts (crunchy) and bamboo shoots, peppers, broccoli, carrots, peas - whatever he likes but just more of them. Again, brown rice. You can flavor the rice with a little chicken or beef stock instead of some of the water, or add a dash of soy sauce while it's cooking. All these things take much longer to digest, so he won't be so hungry in an hour.
Absolutely get rid of the computer except for homework, and monitor that part of it. Unplug the TV. Period. Take walks together as a family. Work up to greater and greater distances. Make it fun and something everyone is doing, not just the overweight child. Make sure he gets together with friends, doesn't just text or email them. Walking home from school is a good idea - no more rides for distances of less than a mile. Put up a basketball hoop - he can start by shooting free throws and graduate to more and more movement to retrieve the ball, dribble, and so on.
See if his school has an open sport - our schools have track available to everyone, regardless of ability. Sometimes a "big" kid can be enticed to work on shot put or other aspect requiring some "heft" but then gets involved in conditioning. Our school has seen quite a few big kids get trimmer and stronger.
See if your Y or local sports location has a reduced membership for kids who can just use the pool. Also, weight training is key - get a set of weights and a really good video - he can start by watching TV if he's working out at the same time. Once he learns proper technique (lower weights but more repetitions - sets of 12 with a rest of 1 minute in between), he can do this even if he is watching a regular show. Get a mat and have him do crunches while he is on the floor. Have the school phys ed teacher show him a workout and develop a chart where he can keep track of his progress. Reward him with a NON-FOOD treat - trips to museums involve walking along with learning. Trips to the beach involve hiking along the shore too. And so on.
This is a nationwide problem so you and he are not alone. Have him invite a friend for dinner and let them make it together - you'd be surprised how long it takes kids to slice up veggies for Chinese food - and if they nibble while they slice, so what?? Have them start the rice and then slice/dice while they are waiting. Then after dinner, send them both out to shoot hoops or hike around the block - tell them it's either that or they do the dishes. They'll disappear!
Good luck. Be firm and be a good example in your own health too!