Okay, I just need to clarify I few things. I really do have the best intentions for my son but I feel so stuck it's very difficult 
You need to look harder at why your son is so different in a different household
As I keep saying, he isn't any different in another house beyond the way he's acting around other children. Which is going to happen since we don't have any other children in ours. He doesn't approach other children at nursery or go near them. But he's a different child at SIL's in the sense that he will follow the children there and tolerate them playing alongside with him, which is a Huge achievement for our DS
H is really close to DS. In the sense that other than me, he's the only person he trusts and will behave like himself around at home beyond me. I feel horrible at the thought of taking DS from that. But I understand H often doesn't have the time of day for DS. And is extremely annoyed by him. But DS doesn't see that (yet) so he's his bubbly self regardless and just thinks his dad loves and treats him like me
I suppose I could list quite a few things I love about H but they are what should be 'normal' in a relationship so they'll only get shot down. I understand that. I always liked that he's never lied to me (as far as I'm aware), never come up short with a lie. Never been dishonest to me. I know I can trust him 100%. Which should be a given in any marriage but I see so so many friends and family with men that lie to them constantly
Obviously I feel inclined to let the C word from this morning go since he's apologised and does seem very guilty about it. But he isn't phased about how he has spoken to me.
And, every day, there is a very clear tone and feeling that I'm expected to take the heaviest load of all in terms of parenting. He's joked a lot before about being 'leader of the pack' and 'the man of the house' but said it as if he was pissing around. He's never said that to me with a straight face. But his general tone makes me feel there is a lot of truth in it.
I think he genuinely loves me but clearly doesn't find me funny or interesting as a person. Rarely asks for my opinion on anything (world topics etc).
I think we would be better off separated by financially I can't see the light yet. I don't have another job yet. He doesn't own this house. My name isn't on it. I don't have family to live with. If we were mortgaged I would be far more inclined to separate as I know I'd get half and could start up new without such huge uncertainty.
My mum lived in a hostel with me when I was a little girl and I still remember now as a mother myself just how brave she was. I remember thinking it was all some big adventure but looking back, she was often at breaking point