Thank you so much for all your replies.
Hmm, not sure what to think really. Instinct tells me that they need to give it a bit longer before they decide its not working.
I asked him about it this morning and he said he just slides off it. Its the Move n Sit wedge they are trying him with at the moment.
Indigo, does he need someone to show/teach him how to use it rather than just being given it to sit on? His teacher seemed to think she could just get him to park his bottom on it and it should work if it was going to iyswim.
She says he seems to naturally want to pitch forward and lean on the desk when he has to write, so the wedge just tips him even further forwards. Generally, when he isn't writing, he seems to need to hold himself up with his arm, so is sort of leaning back holding himself up with one arm, which then gets tired so he shifts his position, same thing happens to the other arm and so on. End result being that he is never still in his seat. He never sits when he can lean and never leans when he can lie down iyswim. If he has nothing to lean against he will either lie down if possible or squat on his haunches (he even squats with his feet on the toilet seat
).
We received the observation report from the inclusion lady this week and it was quite hard reading. Obviously I know he's a wriggler and sitting to a table is a problem for him, but I had no idea how bad he is in class. Quite frankly I can't believe none of his previous teachers have ever raised it as an issue (he's in Year 4 now). He does wriggle about and hold himself up etc at home with me when we are doing homework and eating etc, but he's nowhere near that bad, which leaves me wondering whether there's an element of sensory stuff to do with his class environment involved.
Incredibly, despite apparently being in constant motion and not being at all engaged in the lesson, when his teacher cued him in and asked him a direct question, he knew the answer straight away. So he is taking stuff in, but not fully engaging himself with what's going on. I guess that's how he's always been able to pass tests and achieve good overall results.
Apparently he regularly spends time kneeling on the floor with his elbows on the chair and forehead on the back of the chair. He slides on an off the seat under the table, kneels up, crouches on his feet and is in constant motion. He also fiddles and taps all the time and constantly drops his pencil - or worse, ends up flicking it across the room. She observed him for 45 minutes and the amount of movement she recorded was unbelievable. Its a miracle he achieves anything with all that going on.
Both the school and ourselves completed the Conners scale for him last term and he came out as definitely not ADHD. I don't know a lot about ADHD, but that did feel right to me at the time, as he has an incredible ability to concentrate if the environment is right and he is interested in the work. He can lose himself in a book for hours on end. I assume he would be unlikely to be able to do this if he had ADHD, but like I said I don't really know anything about it.
The inclusion lady has referred him to the ASD Inclusion Team and said she felt that the preliminary ASD dx seemed appropriate from the little she observed of him, but that he also presents as pretty dyspraxic, which is something that has been suggested before.
I really think we need to come up with something to help him be more comfortable and settled in class, but if the wobble cushion doesn't work, I't at a bit of a loss to know what else we could try.
His teacher wondered if a 'kneeling chairs/stool' might be an option if he has the need to pitch forwards anyway, but I'm not convinced.
I am stumped really. Tried to google a bit last night, but drew a blank. Guess perhaps I am just going to have to be guided by the 'experts' on this one. 
I would be really grateful for anyone else's thoughts or ideas on anything else we could try, or things that have worked for other children.