DS is 12 next month and he started secondary school in September. He has always been slim although admittedly did gain some weight during the first lockdown because we rarely left the house (I was pregnant at the time and terrified). I did make him and DD’s do Joe Wicks, laps around the garden, trampolining and yoga YouTube videos but I noticed he’d never put as much effort in as his sister’s so I doubt he burnt as many calories.
Anyway, they returned to school and he lost weight fairly quickly. Partly because we almost always walked home from school (1.8 mile walk) and partly because he couldn’t snack during the day I guess! All fine until he started secondary in September. He no longer walks home because it’s 3 miles away so an hour’s walk and I don’t feel right asking him to do this when it’s cold and dark (he also won’t take a coat to school because there’s no lockers and he doesn’t want to lug it around). DH drops him in the morning and has started dropping him about 15 minutes away so he gets some exercise. He gets the bus home. He does PE twice a week but he doesn’t enjoy sports at all and I doubt he puts much effort in. We have started C25K together this week which he hates but I’ll persevere. He’s also started going to a junior gym session on a Sunday and I’m doing junior park run with him and DD’s to try encouraging him. We usually go on a family walk at the weekend too. That’s the exercise situation.
Diet wise I don’t buy crap snacks anymore at all so pretty much everything in the house is healthy. He does have an issue with portion control though. I serve dinner up so that’s not an issue but he wakes much earlier than anyone else so gets his own breakfast. We only have healthy options but he piles his bowl full until it’s almost spilling over the sides. He also literally spoons mounds of peanut butter onto his toast for example. I’ve told him it’s unacceptable and showed him how much he should be having but he rolled his eyes and thought I was getting on at him. We always have a healthy evening meal fwiw aside from the odd time we’ll have take out or pizza but that isn’t often at all.
My biggest concern with his diet is what he’s eating at school. I no longer have control over this, I used to choose his dinner on the app in primary school or I’d send him with a packed lunch but this isn’t an option now and he isn’t making great choices at all. He has pizza at least twice a week and a cheese or egg sandwich the rest of the week with cake and custard for dessert every single day. He also gets a snack every break time which is a Belgian waffle or toast. The toast may not sound so bad until you hear the fact he puts syrup on top! I’ve asked him to get healthier options because I refuse to believe they don’t have a healthy dessert option for example but he said they only have muesli and he’s not getting that because nobody does and everyone would think he was a weirdo 
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He’s about a stone overweight and I’m worried about his fitness levels. He used to be very fit but now he’s puffing and panting with little exertion. He’s also in 14-15 years clothing, he is quite tall (5 ft 4) but this is a bit crazy, it really shows around his middle in particular. None of us have a weight problem so this is new to me and I’m just looking for some general advice really.