Wow you're getting a hard time on here OP.
That said I'm not surprised, the mantra is very much that SM's should put aside all their personal concerns and act as third fiddle to SC's and parents.
Firstly your DH is unreasonable for not discussing this with you first.
For all the cries here of "you knew he had a son" (obviously she did) we are talking here about a significant change in living arrangements concerning an adult not a child.
This is something that will impact not just the OP but the whole family and it's something that should have been talked about.
Is he going to pay board for example? What are the rules of the house (getting in late after a Uni party when the younger children are trying to sleep)?
Is he doing his own cooking/buying his own food? Is the OP expected to cater for him or not and what impact does that have on use of the kitchen etc
When he applied to uni in London did he anticipate living with the OP? Did he just assume it would be ok or did his father tell him that?
What would he have done if he went to another uni?
Tbh imho part of the uni experience is living independently. I appreciate rent in London is horrific but that's a known - so when taking the uni place why wasn't that a factor?
It's not a simple as some people are making out.
DSD is at Uni 10 mins away from us and doesn't live with us.
She pops over regularly, especially for her favourite Sunday roast (with a ton of washing and leaves with it all clean plus a load of foodstuffs
).
Yes it would be cheaper for her to live with us but we want her to be independent and rather than pay us board from her loan we give her that money to help fund her accommodation (plus loads of help out with food/clothing etc).
She's very much loved by both DH and me but whilst DS is still at school and we are working, her uni lifestyle wouldn't be a good fit for the household and frankly we want her to have fun and not be tiptoeing in at midnight having left the party early and to be in an environment where she can entertain friends
This isn't just about being "mean" or "selfish".
There are a whole host of issues here and the idea that OP should just suck it up without being consulted explains why so many SM's end up a breaking point.