This is a very sticky situation. Unfortunately, you have to follow the last IEP signed and until the parent agrees, it has to be followed by the school.
You are doing the right thing by standing up for the child and I applaud you for doing it. I suggest that you get some peer teachers who may have a rapport with the parent to help persuade them to see it from your perspective.
Data also can be a huge help in helping the parent to understand your position. The case manager (special education teacher assigned) should also help in helping the parent understand.
When you have a meeting with them, wait until the right time to ask (and gently) to the effect what concerns them about this accommodation? Do they know enough about the accommodation and how it is implemented? Is there an accommodation similar that work as a substitute.
Good luck. It is not easy being in your position but continue to make a difference! You're doing the right thing.