The school is responsible for providing the representative members of the IEP team. Your solution will be to advocate for accommodation and modifications that ALL of the teachers need to adhere to. You will need to request a specific goal be included in the IEP in order to measure your son's progress throughout the year and require specific tools to monitor the progress (daily, weekly, monthly, etc.) It is NOT up to the teacher to decide "how it is done" it is dictated by the legal document - the IEP.
Susan