Doesn't sound that out of range for normal to me? Six year olds forget things and this is a life skill that takes practice. I would hope that the teachers are still reminding all the kids to bring in their lunch box. I think we expect a lot out of six year olds in schools these days and if you ask around other parents I suspect his by far the only kid leaving his lunch box in the lunch area or throwing away things that he shouldn't.
Have you had a conversation with his teacher or the principal. What is their routine? Do they have an aide or teacher who watches what gets thrown away in the lunch area? What I remember from that age, was all the kids were supposed to put their lunch boxes in the location where they line up before they go play. Routine is one of the best strategies to teach. One way to not throw away the containers is to have your child not throw anything away. My DD did this for a while and it helped me to understand what she was eating and while it was a bit messy prevented the wrong things from being thrown out.
