N.B.
I truly can't imagine a kid wanting to go sit in a meeting with a bunch of heavy adults who're focused on what they eat, talking about their challenges, and more. I can't imagine a young adult doing that either.
So, here's my thoughts about gaining 10 lbs.
Maybe he's retaining water.
Maybe he had a growth spurt.
Maybe he's constipated after going to camp and getting dehydrated a little bit.
Maybe, maybe, maybe.
I wonder, is the 10 lbs so much that his clothes even fit differently? How can you tell he gained 10 lbs? Do you weigh him all the time? Do you weigh him at home or take him to the docs office and use the same scale there each time? What's the deal about 10 lbs?
I can't imagine it mattering so much to me that I'd want to make my kids go to adult meetings. I would likely not buy breakfast cereals for a few weeks or not make cake or brownies or something. I'd just cut out a few things and see if it made a difference.....
Again, not that I'd even notice of one of the kids gained 10 pounds. I'd likely put it off on an upcoming growth spurt and just ignore it.
Perhaps you are very focused on weight for a reason? I am overweight. The grand kids I'm raising aren't. My daughter is. She was thin until a couple of years ago. She also used a lot of drugs in the middle and was a walking skeleton with sores for skin. So her body has been through a lot. When she got clean she gained over a hundred pounds and has a good life and she cooks pretty darn healthy. Still, my point is that if one of the kids gained 10 lbs I wouldn't even notice unless they came to me and said they couldn't fasten their pants or something.
But I expect things like this. Kids grow and before they shoot up they will pudge out. Their body will go into a stage where they eat and eat and eat. They pack on a bit of padding then they shoot up one night while they're asleep. They wake up and the clothing they've been wearing is too big around now and too short.
This is how their bodies work.