Is he really not getting any extension work? He sounds so bright. Actually I sometimes think schools don't really know how to challenge very able pupils. They are often very closed in their thinking - our school would never move someone out of year group because there are lots of bright children and they think they are getting sufficiently stimulated.
Last year DS2's extension work consisted of choosing chapter books from an older class - but he didn't get to do any lessons with them. DS2 is bright, particularly verbally, but nowhere near as bright as your DS (he is not especially good at maths), and he is very young in his year which helps. If he had been born a day later I think he would really be struggling. As it is he says he hates school almost daily.
We do not have the ASD issues, but we have had inappropriate behaviour - biting twice, for example, which he has never done before.
I was observing a Year 3 lesson a couple of weeks ago, which was with a top literacy group (they stream across the two-form year group from Yr 1) and the teacher was re-capping on the use of the apostrophe. Got home and checked with DS2: "When there's a letter missing, like I'm, or when something belongs to someone". That made my heart sink. He is already coming home and telling me what spelling mistakes or other mistakes his teacher has made.
Do you know what kind of work would make your DS happy? I know that is a silly question, but while I realise DS2 is bored, I don't know if he would be less bored with harder work. What he wants to do is "scientific experiments", not sitting down and writing recounts or recipes.
I think the school see DS2 as one of a number of bright children, and perhaps that is all he is. I really have lost of sight of whether it is normal for 5 year olds to use words like 'penultimate', 'immortal' and 'temporal' correctly, or to use negative numbers in their number sequences.
I really hope you have a productive meeting with the school - your son is clearly a wonderful and talented boy who needs lots of stimulation and understanding.