I can hardly believe this is real, but if it is, you need expert help ASAP. A general ped doesn't count. He's clearly eating too much and not getting enough exercise, but he also needs a full check-up for metabolic diseases, and both he and the rest of the family need to be coached on weight loss, exercise, and nutrition. You don't say whether you've had regular conversations with him about the effects of food on his health. Is anyone else overweight in the house?
Ask this board, and the doc, for a referral to a weight/fitness program for children - I am sure there are some, perhaps at John Hopkins or Children's Hospital. John Hopkins has a program for adults consisting of a team: doctor, nutritionist, psychologist (and I forget who else, maybe a sports doctor), and they are supposed to be very good. Hopefully there is a similar program for kids in this area!
Empty your house of junk food and practice healthy portion sizes. Junk is fine on rare occasions, but it's easier to control if it's outside of the house, eaten as a special treat. Also, he needs daily vigorous exercise. If you don't want to run around the block with him, after school, can you enroll him in sporty after-school activities for every day of the week, at school or with businesses such as Dynamite Gym or martial arts dojos?
You can turn this around, OP. My best friend at that age was overweight and couldn't run as fast as I could. A year of intense martial arts and better nutritional choices later, he was WAY faster than I was: I remember being so jealous