Rejected petition Son in year 5 class but year 6 pupil forced into leaving

My son was very academic, we had worked hard for years to ensure a fantastic education. He was in a horrific road accident, left for dead in 2011. And doctors predicted he would be a vegetive state forever.
We united as a family and helped rehabilitate our 8 year old son. He was placed into a special education school for 2 years where I say him at p8 level and learn not a lot. He wasn't interested and he didn't fight. I decided to fight for mainstream school and we won a place at our local primary. He was placed into a year 5 class instead of year 6 to help him work well. His education in the last year has flourished, learnt to write so he can communicate now, gone to key stage 1 & 2 and met some amazing companionate friends. He is at last happy .
We want to hold him back out of cohort but the head teacher is declining , this is tearing our family apart and our poor boy is going to loose his friends as well as the progress .