Don’t put him on a ‘diet’ per se but make sure he’s filling up on healthy foods and snacks. Fruit, cheese, yogurt etc rather than crisps and chocolate. Cook healthy meals, don’t rely on processed ready meals and make sure there are lots of veggies to fill him.
Encourage him to move more, go for walks as a family, have a kick-about with a football, go swimming, if he’s embarrassed by his weight go to an empty beach and swim or buy a body board. (There are plenty of deserted beaches if you know where to look.
13 stone at 14 yo, is too much. Yes kids are bigger, but they should be bigger, stronger and healthy. As his step mum I presume you get to play a part in what he eats. So as the responsible adults it’s up to you to make sure he reaches for a peach rather than a packet of crisps. So don’t buy the fattening snacks and he won’t be able to eat them.
You need a blanket ban on unhealthy crap, so no more cola be that full fat or diet. Even calorie free coke causes spikes in your insulin levels, try sparkling water if he doesn’t like tap water. Even Apple juice is very calorific compared to the zero calories in water. No more chocplates, biscuits, crisps, lunchables etc. If it’s not in the house, he can’t eat it.
Cook meals from scratch, that way you know what goes into them and no more takeaways.
Eat less, move more! Could you consider getting a dog? On the understanding that SS walks the dog three times a day, ten minutes every morning, 45 minutes after school every evening and ten minutes before bed then you will buy the food, pay vets bills etc. (This only works if he really wants a dog)
Or take up a hobby like kickboxing or Thai Che.
You’re an adult, I’m sure I’ve not told you anything you didn’t already know.
None of the above will make a jot of difference if his mum and grandparents keep killing him with ‘kindness’. Is he aware that he is obese?