Ahhh the summer of Fortnite, right? My 12 year old is kind of chunky too so I totally get your concern. He was the smallest and thinnest of my kids and started getting heavier a few years ago. Right now he could easily stand to lose 10-15 lbs. He does play hockey and lacrosse, so that helps a little, but it's not enough. I see my son getting into a cycle where it's a little harder to move because he's getting heavier, so he's more likely to be sedentary, which makes it easier to gain weight, and so on. Honestly over the summer on the weeks that he wasn't in camp I didn't have great plans for him so it's been a TON of gaming and TV for him during the day some weeks.
Once school starts, we limit video games to weekends only but then my kids can just watch TV or are sitting doing homework, etc. (at 12 & 14 they're on their own after school except for sitters who mostly just drive them places) so it's not like they're off running around the neighborhood after school, burning calories. And we live on a busy street so they can't just ride bikes or walk anywhere either, but he can shoot pucks or play basketball in the driveway or toss a football with his brother in the yard.
So...I'm in the same boat. I think he's starting to understand that his choices to be sedentary are affecting his ability to compete in hockey and he's getting self-conscious about his appearance, always wearing a swim shirt in the pool, for example. The next step is to put those feelings and recognition into action and choose to go outside, or stay after school and use the gym (they have a late bus option), etc. In the meantime, we talk about healthy portions, about easing up on treats, and try to go to the park at night, ride bikes, and get out and active on weekends. I don't want to shame him into trying to lose weight (didn't work for me!) but I do hope that a growth spurt and some competitive fire for sports will help him to slim down a bit.
It's hard to accept that this just might be the weight he's going to be for now. My parents make comments to me and it drives me crazy. He's not so heavy that he has health issues, and of course I want him to look and feel his best, but to me, it's not worth making him feel bad because he's not as slender as his emaciated siblings (my other kids are all rail thin).