Just as it says but a good deal of additional information:
My DS is 12 and is in specialist school and has a couple of different diagnoses. One of these he shares with this boy (who is in small nurturing mainstream) and my DS has zero local friends (I do mean zero - he's in a class of 5 at his school and seems to get on with them, but they all live at least an hour away, some are in care, some like him struggle with socialising, we aren't going to get together with these friends). He's now in a sports club he likes, and he loves swimming even if the session is noisy, so he's getting out and active but doesn't have any peers he sees. Ever.
Mum of friend and I are occasional coffee pals and mum has started referring to "Jake" as "they". Local friends who are GC and know this family say that boy may be distancing himself from possibly abusive ex of mum (Jake's dad). My DD (10) also knows this boy and says "how can he think he isn't a boy or a girl?". Mum knows I'm GC due to discussions we've had in the past, but we're cordial as we share other things in common, and she's said that Jake would like to meet up with DS over the school holidays. (as an aside, mum's message confused me as it said "they would like to see them" which may be her refusing to use sexed pronouns for anyone... or getting in a tangle with her grammar... or possibly meaning that Jake would like to see both DS and DD??!)
I feel that getting to know this boy again (they were in Reception and KS1 together and saw each other at clubs till lockdown) would be a step towards socialising, we have absolutely no other families willing to see us, even other parents with children the same age only see me separately. But I'm also worried about DS reaction to anyone referring to the boy as "they". DS spoken language is OK but not very sophisticated - he sometimes muddles his word order or grammar - and I am trying to work out what to say when DS either a) gets confused about who's being talked about or b) says something blunt!
If you were in this situation would you prewarn DS (he may not even notice) or just let it happen and explain at the time - which I would feel awkward about - I'm not going to say "oh Jake likes to use they/them pronouns" as DS would be extremely confused by that, nor am I going to say "Jake's mum can't use English properly but you just have to call him, him" or even "Jake thinks he isn't a girl or a boy but we know he's a boy so just call him, him" which would at least have the benefit of DS understanding what I'm on about.
DD totally gets the nonsense that is GI and is happy to go with "yes just like Aunty Amy is very large but we don't say SHE'S FAT to her face, Rowan thinks he's a girl though we know he's a boy and we don't shout YOU'RE A BOY every time we see him, and Jake thinks he's neither but there's no point in telling him he can't be neither" - but DS has the bluntness of many neurodiverse children and will happily shout IS THAT A MAN OR A LADY on the bus.