Triggles/Gottalovecosta, the LA wanted SS for DD1. I hope that it was partly to do with my summary of her needs.
I told them: "Whatever you do with DD1, it is going to be expensive. But she's worth it. If you spend lots of money now, you might have to spend less later. She's a wonderful girl who just needs a lot of help to understand her world."
Try to think of things your DS needs which points to SS rather than MS. Things like whole school communication. EG. At DD's school every member of staff uses Makaton at all times, regardless of the level of verbal ability a child has. DD1 can speak (she has S&L delay/disorder, but is verbal) but she resorts to signing at home if she's very stressed. Just by increasing her signing ability, her stress has decreased because she has the option of speaking, speaking and signing, or just signing (she rarely 'just' signs, but sometimes).
Another thing that DD1's school does is to do with language. The SALT does communication groups, then the teaching staff use the focus of the group to reinforce it all day, every day.
DD1, despite her learning difficulties is very clever and quite socially manipulative. She got quite dependent on her 1:1 in MS preschool & they ended up letting her dictate to them. At the SS she is in a class of 9 with 5-6 staff. They can encourage independence without sacrificing a high level of help.
Think about sensory issues, too.
Basically, SS is expensive. It is by far cheaper to send a child to MS with 1:1 than SS. So think of the things that make SS more effective.
Also, think of the reverse angle. What makes MS 'incompatible with the education of other children.'
If you want SS, then refuse to have your child excluded unofficially. Let it be the school's problem. They'll have to formally exclude him. Paper trail.
The big tipping point will be behaviour, generally. A placid child who is 4 years behind isn't as big a problem as a child with behavioural problems who is 1 year behind!
Stick to your guns and play the long game.