How hard is it to move my child from a special school to mainstream?

My child has attended a special school all primary (ASD was nonverbal but now fully verbal). He now has a place in a special school in high. However, lately I am feeling that maybe the special school is now holding him back socially and academically. Academically he is behind but this is because he doesn't show his ability in school and isn't challenged enough or taught the curriculum as needed. Of course, I am grateful for the tremendous support he has received when he needed it and know so many families struggle to get a placement. However, having met the secondary school, they don't offer GCSEs and only FS in some subjects. Also, socially my child has struggled (outside school he socialises with children from mainstream but the gap is getting wider as he doesn't get enough social opportunities with similar children to him) and I feel it is affecting his development. I am torn between special school and mainstream. Special school is no longer a challenge and mainstream may be too much. However as I am thinking about this, please could someone help whether it is possible to move him to mainstream if the first year of high isn't suitable for him? We don't have any mainstream schools with sen units near us which would have been better. Its either or.
What is the procedure and what do you think is the best way to deal with this? I know he wants to go to college and I don't want to hold him back but I also want him to safe and happy and do what is best for him.
School said when I asked he isn't ready for mainstream but also admitted that he doesn't show his true self in school and have seen glimpses of him that suggests otherwise when teachers are not around.