Tall guy? Does not need to lose weight. Just from looking at him he is in no way overweight, his BMI (notorious for not differentiating between muscle mass and fat) would be a very healthy 19.5. He does not need to lose weight.
And sweating wouldn't help him lose weight either? Yeah you can "weigh less" if you sweat, but that's literal water weight, you get it back when you drink water. And if you're burning fat you SHOULD be drinking water.
Chubby is exercising, trying to be stronger, faster, get more endurance. He's trying to build muscle. He needs protein. I don't care what anyone says, you can't convert fat - stored or otherwise - to protein. Let that kid eat!