It's about changing the way he THINKS in order to change habits. It has to make sense to him and understand the benefits. It's an EDUCATION! I've been teaching nutrition for YEARS....starting at TWO.
There is a book called HELPING YOUR CHILD LOSE WEIGHT THE HEALTHY WAY by Judith Levine, R.D., M.S. and Linda Bine
You need to start including him in grocery shopping, meal planning and making good choices. DO NOT allow things in the house that will sabotage him.
Pick something new EACH week from the produce department. DO NOT allow your negative opinions about something to influence him. ENCOURAGE ENCOURAGE ENCOURAGE!!
When you try new foods, research together and find out WHAT nutrients are found in the food, WHY does your body need it, etc. It might require that YOU have to take some time to learn too, but it will be WELL worth it for EVERYONE!
If he doesn't "want to", then he also should not be allowed something he "wants" whether that be computer time, cell phone, video games, etc. TALK ABOUT BALANCE IN LIFE and how it is important to get things done that NEED to be done, NOT just the things you WANT to do.
Research things on childhood obesity and talk about them TOGETHER. Don't just throw a copy of something on his bed for him to read.
If you have to go to the extreme of taking him to a doc and having the doc TELL him and SHOW him how he's taking YEARS off his life, stressing his organs, etc. THEN DO IT!
I was 120# 4th grader w/ a thyroid problem. I KNEW I didn't want to be on meds the rest of my life, so even at that age, I wanted to know more! Somewhere around 13, I was "normal size for my age" and have only had to go back on meds ONCE, when I was in college. I've learned how to control my metabolism thru diet & exercise.
I'm 52 and weigh 126# and won a Ms. USA bodybuilding championship at age 42. I competed on the USA bodybuilding team at age 45 and was in the best shape of my life! TRUST ME! It's worth the time and effort. It's about QUALITY OF LIFE!!!
Set goals, too. What does he want to do? Like learn to water ski, etc. Have him carry around a 15-20# dumbbell all evening long. I do this with clients during their workout time. They tell me how hard it is and how heavy it is. I then remind them that's how much difference 15-20# makes (make it 10-12# if you want) but I remind them that this extra weight is taxing on the joints, organs, respiratory system, etc. The MORE SENSE it makes, the more he'll make the effort. Do what you have to to help him and educate him. When you set goals, set long & short term ones. Remember that meeting your short term goals gets you to the long term ones!