To move my DS year 10 who has an ECHP from a mainstream secondary school whose target grades are around grades 2/3 to a Private School and ask DS to start year 8 in January at the Private School to a Private school and ask him

Could your son not move to a different private schools that’s more on the nurturing/small year groups. Maybe to restart year 10 next year and focusing on tutoring/extra-curricular to help him get some life skills along the way?

You can’t do that. It wouldn’t be fair on your DS or his 13 year old classmates. Kids don’t have to stay in education until they reach the end of school, they need to stay until full-time education until the last Friday in June of the school year they turn 16 (I’m in Wales, double check if you’re elsewhere) so he could leave in year 9!

You need to accept that he is different to his sisters and unlikely to achieve the same academically as they do. What are his strengths? Find a way to focus on them rather than exam grades.

I teach mainstream state secondary. We have a handful of students educated out of year due to SEND or coming to the UK from a different school system. One year below seems to work, especially if the students is one of the youngest in the "correct year". However, year 8 and year 10 are worlds apart socially and developmentally. In many private schools year 8 would surely be "prep" age and so there wouldn't even be older students for him to socialise with. I agree with other posters that you need to look for a school with a more nurturing, less academic, environment.

I wouldn’t be happy with my year 8 child with a year 10

Ok I am very confused

there are 2 DDs yrs 11 and 9
adopted (what age) year 10 DS who is currently at state and DH wants to move him to private?

Fellow adopter.
This sounds an unusual approach.
Your DS has needs that require an EHCP, and is currently predicted 2s and 3s. You want to move him back 2 years to a private school that has very high workload expectations.
Depending on his needs, would he really cope with that? Neither of mine would have.

I'm possibly looking through the glasses of my own two ADs, but I worry you are trying to educate a less academic version of your DDs, rather than the DS you have in front of you. Going back 2 years could have massive social implications going forward, though at least with the EHCP you will be less constrained at college level.

Would it not be better to work with the current school / tutors, to try to get some of the grades up to 3s/4s which would then allow a level 2 course at college, followed hopefully by a level 3?

100%

Find a school that offers a nurture pathway, perhaps with a SEND resource base.

Academic success is not the most important thing in life. His happiness is. Who would be happy with 2 hours homework per night?

So this does not sound like the right school for him? They’re very focused on homework and academics- it’s nothing to do with emotional maturity?

It seems to me to be putting the academic goals ahead of the child’s needs. Moving two years down in school is not a child centred approach.
Stop comparing him to his sisters. Some children, EHCPs or not, are can make amazing lives for themselves without the academic measure of GCSEs. Other than maths and English, or for some jobs science, no one gives a toss what GCSE results are once you’re older.
invest in a tutor and consider your son’s needs - including social needs. What are his strengths and interests? What does he enjoy doing?

Are you expecting DH or his family to pay for this private school that they don’t want for extra years?

Terrible idea.

you can’t move a 15 year old into a class of 13 year olds- they’re a world apart developmentally. And imagine how that would make your Ds feel. Being demoted to the year below his little sister.

if genuinely this is the right school for him and he wants to move then year 9 might work but it would need to be done very carefully.

I think without knowing the schools or your DSs needs it is going to be hard for anyone to give an proper opinion. But if the move is best for DS and you can afford it then I would do it.
Do you think it would be best for DS, or are you just annoyed with your DH being dismissive of his education?

Is DS independently financially secure? A private school sounds good idea but why year 8 and not year 9? 2 years back might be hard socially.