Welcome!
Sometimes it helps if the previous year's teacher is able to add a recommendation for advanced math, based on his performance in 5th grade. Have you received any feedback from that teacher that would be helpful or relevant?
In terms of placement policy, most state guidelines allow local districts to make their own; they are not necessarily binding on local districts. This may or may not be the case in your state.
Whether this class would be appropriate for your son (aside from school policy) depends a bit on the nature of its "special"ness. Perhaps it would help in advocacy if you were able to demonstrate that his skills match the prerequisite math skills for the class, and that his educational needs match the offerings. It may be that some of the data you have from CTY, past aptitude testing, and lists of specific skills mastered in Khan Academy may be useful for this.
In general, school communication and advocacy is most effective if one is able to keep discussions at an objective, task-oriented level, focused on the needs of the child, possible solutions/available resources, and the best match between them, and leaving personalities and emotions out of it.
