As a former K teacher, I can say this has happened in my class. Especially if the lunch somehow ended up not in our basket - I took the kids to lunch every day and for the first few weeks I got them sat down and eating and if someone couldn't find their lunch box I would assume they forgot they were supposed to eat hot lunch or forgot to bring it at all. And the lunches were often accidentally put back in the wrong basket. Lunch can be chaotic. But they get the hang of it.
Email his teacher, she can help him remember what to do at lunch time.