I wouldn't put him on a diet. That would cause problems later on. But if you are all overweight, then you need to be honest with each other about why, what impact it's having now and what impact it'll have down the line. he needs to drop down to around 2000 cals so cut down his intake by 50%. That will feel like a massive change. The least noticeable way to do this is by cutting down on fat.
At that age he could shoot up and in a couple of years time that weight will just be a bit hefty, by 18 it could be normal. But he needs to stop putting weight on.
Can you focus on the whole family trying out healthy living for a month? or even for a week? Do something together for an hour a day - swimming, cycling, ball game, water fight, gym, a walk in the woods or a night walk in the city etc. Don't worry at all if he resists. Tell him it's normal not to want to but you're going anyway. Remind him the reason for doing it is you love him and want him to be fit and it's your job as a mum to help him be healthy.
If you do a health kick it doesn't need to be radical yet. If it is, they might reject it. You could start with lower fat foods they already like: grilled fish fingers, oven chips, peas, reduced sugar ketchup. Make sure he drinks lots of water all day long. Make your own burgers with lean beef (just mince, moulded into patties and grilled) with baked potatoes and lots of salad.
Buy mini milk lollies or juice lollies as a sweet treat for after dinner. They have less than 50 cals but it cheers up DC of that age to know that losing weight or getting healthy isn't a life sentence of lettuce and brown rice.
Give him sugar free gum to chew between meals.
At the end of the week ask if he feels better. Chances are he will feel slimmer and have more energy. Those are good incentives. Give loads of encouragement and lots of non-food treats for each week you all keep it up (new CD, new phone app, new tee-shirt, get a film he's always wanted to see etc.)