It sounds dodgy as to me. It's the someone she knows who can sort it all out bit that really red flags it for me.
mentioned someone she knows has had advice from a solicitor already and will sort it out and set them up with somewhere to live?
There is also concerns about her age she said she is 26 but her passport says 29, she told my son it was because who she worked for in China changed it!
This sounds like the biggest load of cobblers and a forthcoming sham marriage scam.
Your hands are a bit tied simply because you are his Mum so he doesn't want to listen to you.
Who will he listen to? Is there anyone in his life he looks up to and respects and will accept advice from?
Can you contact the Foreign Office and explain your son's circumstances and ask about legal ways this girl could stay in this country. Ask them what SHE needs to do, rather than having your son possibly duped.
If he does marry her, she'll be entitled to half of everything he has including pension, savings, inheritance etc. after a short time no wonder she's trying to rush him into it. See your solicitor and see what you can do so she can't get at it.
I know this is only hearsay, but a friend of the wider family is Chinese and about the same age as your son's friend, her parents have never lived here, she was in boarding school here then Uni here and she's recently become British (a British Citizen? not sure of the terminology but she can live here) but she has NOT got married to do that, so there must be a proper legal way for Chinese girls her age to do it.
my son says if I don't accept her then she can't marry because of her culture
I haven't got a clue about this but would be finding out from official sources very quickly.
It looks as though both of you are being railroaded into making decisions that only one person will benefit from - her.
That's not how love or family work.
It is how scams work.
I'd also consider re-posting this question on a busier main board, Legal maybe, or Chat.