Could you see about getting a small container of sunflower butter and see if he likes that? it doesn't taste the same as peanut butter and takes a bit to get used to but the texture is similar. I haven't tried soybutter but that's the other alternative. I just hate to see the lunch money be wasted if he won't eat the school lunch. Would he eat bread with regular butter or margarine? What about those lunchables? They're not all just meat and crackers.

edit: also you can try googling kid friendly sack lunches and see what turns up there for more ideas. There has to be something that he'll be okay with. {hugs teacher and son if okay}