sorry he isnt feeling well, unfortunatly Children will eat anything, we feel bad when we see them eat mud as well lol but on a serious note you do need to complain as they seem to be brushing this off as unimportant, your son gets 1:1 because of things like this for starters and if the 1:1 goes home at lunch there should be someone taking her place (thats what i fought mainly to get matts 1:1 for as i worried about him gettin lunch now at college) and also its a health and safety issue, not only for your son but the fact that it wasnt thrown out at the end of the day never mind the end of the week it shoudnt have even been in the school never mind the trolley.
hope he feels better, perhaps try him with some arrowroot biscuit or a rich tea as they are plain and will help his stomach settle.

corinne