I thought I recognised your posting style.
Oh ffs! A planned event and they hadn't forewarned and prepared him or briefed his support.
Does your ds go to the same school as mine? They certainly sound spookily similar in their behaviour.
We're having a very similar problem - regular supply teachers (as in weekly at present) but no-one telling him in advance and we've now found out that he isn't actually getting any TA support at all one day a week, which just happens to be the day they seem to keep getting supply teachers - and so we just go round again.
Could you do what we've had to do and call a meeting of all professionals involved in his support to discuss the statement and if they are implementing it properly to support his needs, as well as doing everything else that's necessary to support him? Clearly they aren't, as they should be doing basic things like forewarning about change and informing support and adhoc staff of his needs, so you have enough justification right there to call a meeting. Ours is not an emergency annual review, but we have the statementing officer, ed psych that wrote the statementing report, inclusion teacher and her boss, senco, head teacher, ourselves and our advocate all attending the mother of all meetings to try and get him as supporting as we possibly can so that he can make it through the rest of the year.
This year has been a nightmare so far and ds is a mess, but he still wants to attend at the moment and we are fighting to make that work for him.
Dh has been tapping away on his laptop all night preparing notes for us to use at the meeting and will likely be doing the same all tomorrow night - doubt we'll get through it all or that they'll see things our way, rather than closing ranks - but we're going to give it our best shot.