"I've had another chat with dh, who will not budge. the best I've got out of him is that if ds is unhappy we can take him out of school, but he still wants him to start this Sept."
I know it wouldn't be your number one choice, but that is not such a bad result so far. If your dh is as nervous of bucking the mainstream as you say, it is quite brave of him to consider the possibility of removing his son from school "just" because he's unhappy. It sounds like your discussions are bearing fruit.
There are a few things which might be worth keeping in mind if you and your dh decide to go with the plan of trying school and removing your son if he's unhappy.
Might he lose his school place if that happens? The place will be held for him if you defer his start without sending him in the first place, but if he goes to school and then comes out, I'm not sure he can keep the place if someone else wants it. (Of course, if he absolutely hates school then you might not want the place anymore. But would you have the option to remove him from school for a couple of terms before sending him back in the summer?) If this is a concern, maybe it would be worth writing to the LA so they can clarify the situation.
How long would you give it? How unhappy would your son have to be before it is time to take him out? Many people who are wavering about school send their children with the idea of removing them if it doesn't work out. But once the child is at school, if things don't go well everyone tells them to give it just a little longer, and a little longer: "it's just settling problems, you can't expect them to have adjusted in the first term, don't be seen to be giving up, don't waste the energy you've already invested in adjusting to school, you'll only have to go through all this again later if you take him out now, keep him in because maybe he'll be happier with the structure in Y1 or a new teacher next year..." It can be a bit of a trap, being in an environment where everyone else is committed to school.
It could be helpful to chat with your dh about how much unhappiness would be too much to make your son go through before deciding to change courses. Is it acceptable if he is exhausted and difficult at home but performs well at school? If he doesn't complain, but simply loses his spark? If he cries every morning about going to school but then cheers up after he is there? If he says he likes school but he starts behaving quite badly? Then you'll know whether the two of you are on the same page.