I would tread carefully for several reasons. This is a private school and they do not abide by IDEA at all (they are not legally required to) only ADA. ADA is an access law and that is all. You can take your IEP and throw it out the window. If they want that class to be required for every student it 100% can be. You will need to think about this differently and be prepared with what your son would need to access the material in the third language class. This way, if they do require it you are prepared to advocate for your child. Out of curiosity, how did your son do in the first two language courses? What makes the third course drastically different? Also, why was the diagnosis by history? Have the previous deficits been remediated?