One of mine did this and it turned out he was treating his friends after school but before the activities. (None of the kids came from homes where money was tight.)

He was also eating a lot and although he grew a lot in his early teens, he did end up having a weight problem which took him a long time to get control of.

Whether you can afford it or not, it's not ok for him to agree on one thing and then go off and do the exact opposite.

Maybe try a week of hot lunches only and see how he likes that. Throwing away food (or letting it go to waste) shouldn't be an option for anyone.