Paperwork in place Boomer.
Copies of IEP's and meetings and consultations with tutors, head and SENCO, sped ed teachers. Evidence that they have/have not met his special educational needs and possible admission on their part of why they are unable to meet his IEP, whether thru resources, time, teachers, specialist teachers.
Without the evidence and the back up from the consultants that this particular form of education has not met the "every child matters", or equivalent thereof in the US, then you are fighting just your feelings. You need their policies, county policies, the procedure is complex to prove - but it can be won with grit and determination. Only focus on what is best for Mo, in your opinion. Ask for advise from those who have experienced this in your area.
The school may be able to show their considered progress and have results to prove their intent and ability to educate Mo, and the special ed team may be able to point out reasons for this, or possibly admit that the targets on the IEP cannot be met in mainstream education for your particular child.
Paperwork and copies to back up your understanding and wishes of what you feel his needs to be, what you have done to ensure his needs are met from your parental responsility, and evidence of where.why the school are not meeting the IEPs and structured learning environment regarding your particular child.
Line you paperwork up in chronological order. policies, dates, requests, meetings, minutes, consultations, IEP changes, school reports etc...
Ask questions and get educated answers. If these don't meet the polices within the school and you can evidence this, then you ca go to appeal it.
Your process is different to ours, but the basics in spec ed are the same. An IEP is set with parental and school and psych/angency/consultants etc evaluation and input. If the targets are failing your child's learning abilities or his possibly difficulties in understand mainstream education and accessing the curriculum within that school setting - you have your next step of the journey to go through.
Wishing you all the luck. You are a fantastic advocate for Mo - do what you believe is the best for him with what you have available. Don't give up if this is what you would believe would benefit his education and well-being. Good Luck girl. You are a fantastic Mom - be all that you can be for your boy.
Calm. Focused. Unemotional. Paperwork. Always remind them Mo is an individual and needs to access a curriculum designed for his level of understand and one he is enable to engage with, not just educationally, but socially as well.