Do your son have an IEP? I assume so, since a dedicated aide is a fairly high-level intervention. Your SPED department may have a "wait-to-fail" mentality. I will assume that you are completely correct and your request is legitimate. Please understand this is only an assumption. I don't know enough to form an opinion. I do know some parents make unreasonable demands and requests, but on the face of it your request seems reasonable.
 
If you appeal to the board, whether in person or in writing, emphasize:
• Because change in routine is so disruptive, if your son might end up at the traditional schedule school, it would be less disruptive to start him there.
• Your son's capacity for coping with environmental stimuli diminishes over time. It is as if he has a limited amount of "sensory stamina." Long school periods, long school days, and long school years all stress his stamina. Limiting the length of the period will help him persevere.
• Since the length of the block restricts your son's learning, that school is not the "least restrictive environment."
 
I recommend you find out who your district's "compliance officer" is. Use that title specifically, since both IDEA and §504 require school systems to designate a person as compliance officer. Contact that person and request things like your notice of parents rights, contact information for the Office of Civil Rights of the U.S. Department of Education, any district policies related to placement of students with special needs. Make your request in writing, and copy the compliance officer, the director of SPED, the superintendent and the president of the school board. Be very polite but do not defer. You are the world expert on your son, so don't cede your expertise to anyone.
 
Principal Peter