Is his plan a 504 or is it an IHP? If it is a 504, then this is a 504 violation. I would document it in writing. I cannot even count the number of times I and others on this board have been told food is safe for our kids when it is not. Dd was told cookies did not have milk and when she read the label, butter was listed. One person on this board had a teacher tell her child a dessert was safe for nut allergy and it clearly had a nut warning. I would not take teacher`s word that it was safe, if your 504 says that you are to be informed in advance. I would write a short letter to the 504C, something like "Dear ____, I wanted to let you know that ds`s 504 was violated on ___ (date) when macaroni was used in a classroom project. Per his 504, I am supposed to be notified in advance if food is going to be used. Thank you for your attention to this situation. Sincerely, ____".
On the other hand if he does not have a 504 or IEP and his plan is only an IHP, then I think you are stuck. If he has a 504 or IEP, I would nip this in the bud. Seriously. If the 504 indicates that only you can decide if the food is safe (by requiring that you be told in advance), they need to follow that. The next time it could be something with bigger consequences.