Thank you both for your expertise and comments, and especially working for the SaLT input. It always astonishes me how willing people are to contribute here. The only way I could ever reciprocate is in the unlikely event of either of you ever needing a crash course in medieval Latin or femininity and masculinity in the 12th century, so thank you so much.
I think (correct me if I'm wrong) that practically I should be asking SENCOs more precisely what they do, how far they tailor it to his individual problems, and what the involvement of SaLTs is. I got the distinct impression they basically thought SaLTs should sort out speech and then he could get on with MS methods. My difficulty is that I don't really see that as fair, he can't just hang around as the rest of the class do sounds he can't make, so what will he be doing at that time? Because, to be honest, if that's the approach I'd rather keep him out of school and in nursery/speech therapy than send him there.
The other problem is his inclination and how we parent. He is fascinated by words-often asks for them to be written down-and is spotting patterns both in sound and on page (last night I misread 'track' for 'truck' in a story and he said 'no, TRACK, it start with /t/ and it got /a/ in mid-mid', then he pointed at the word 'truck' and went (after a pause) 'that not got /a/ that got /u/ for umbrella'. I tend to like it when he does stuff like that, which he picks up on, but what I can't really get across is that while that's what he meant, what he actually said was something like 'hrash' and 'druk'. So he's getting correspondences from adults, but not producing them himself. He can 'read' in some sense, but could never sound to his words.
His speech disorder is, I think, mild in the sense of comprehension (it is easy to tune in to what he is saying, and he has good, if sometimes over posh and adult language otherwise which helps with this). But in terms of production he can't produce /p/ consistently when asked to in isolation, yet alone in words (although he can do so for some words-automatic response which I'm told is typical). And that applies to some really basic sounds to: /b/, /d/, /h/ /t/. And listening varies immensely according to whether he has an ear infection or cold. I don't know how to cope with that, because he will at this rate end up 'reading' simply by recognising and memorising words (as he does now) naturally from stories and questions but not by sounding.
And that sort of terrifies me because I always felt such 'look and learn' stuff was giving him information to work with in the future and to form patterns form, but now fear it may be confusing him more. But I'd feel very sad if I didn't write a word when he asked me to, and I know he feels that being able to say what words are is a useful fallback party trick at times when other adults think he's a bit dim (when unfamiliar adults introduce new vocab and other kids say it his 'tactic' is to look round for a related word he can see in a book or on the wall and go 'that word spell...').
Anyway, your help immense. Any advice on what I should ask or request form SaLTs and schools?
He loved SaLT this morning, even truncated with the hour car journey in the snow. He's playful, happy, full of songs at the moment, and loving to most. Whether he's bright or not I can't say, but I suspect I will love his world the more of it he articulates. I don't want to lose any of this next year.