Thanks everyone, some good advice here. I'm in Switzerland (outs self!). A friend is going to give me the contact details of a lawyer and I will be speaking to them as soon as possible. I have also started sending text messages outlining what we have agreed for the short term so there's no confusion over what has been decided.
I have been a SAHM since DS was born but have recently started to take on some freelance work. I will struggle enormously to support myself and my son in this country on the money I earn doing that. It's extremely expensive here - rent, food, health insurance etc - and my UK based income doesn't transfer well.
I am torn between thinking this is a cold calculated move on his part and thinking that he's depressed/had some sort of breakdown. He says that he has been feeling disengaged and unhappy and that I should've have noticed. His extremely messed up and toxic controlling mother was here last weekend, while I took a break, and I also wonder what poison she might have been whispering in his ear. The friends I have told are extremely shocked as on the outside - and from my perspective - we get on brilliantly and had a good, strong relationship. A couple of years ago he got depressed - there was a redundancy, death of a family member he was very close too, and his DF had a cancer scare - but he had some counseling and felt better. His DF has recently been diagnosed with cancer and so I wonder whether it's too do with that.
He said he'd been thinking about this for months but I've asked him whether he thought about it before we had a holiday 7 weeks ago and he said no. He also seems completely clueless about what happens next so doesn't seem to have forward planned anything. He seems to think he will continue to live in our flat for example.
I just think wtf and why wouldn't you even try? We are not teenagers, we are adults with responsibilities to our DS.
I think it's highly unlikely he has had an affair as he's not really out without me very often. A one night stand could be plausible I guess. I have been through his emails and nothing there.