I also have quite a full on and emotional DS - now 5 and similar in that he is always really well behaved at preschool / school etc. I do think they take it out on the people closest to them and save up the good behaviour for others. I'm also not with his Dad and have a new BF so I know those changes can be tough for them too (not the DV side though - that must be incredibly tough for both of you).
I wonder if you're in a bit of a cycle where you give consequences - loss of toys or naughty step and he feels it happens so often he just doesn't even bother or is deliberately naughty as he thinks "what's the point, toys will just get taken anyway". I think I'd just tell him you won't be taking toys away for the rest of the week. The removing them doesn't seem to be working so what have you got to lose? If he spits on them, just say neutrally "we don't spit on our toys, here's a cloth to clean it". I'd also do lots of sitting on the floor playing with him, chatting to him etc. As much positive attention as possible ( although that must be hard with a baby too).
Also, lots of love and cuddles, even if he has been really naughty all day. We always do stories and cuddles before sleep so that the day always ends on a good note. I never use them as a threat/consequence.
I've recently bought this book: www.amazon.co.uk/dp/B007VDHAP2/ref=dp-kindle-redirect?_encoding=UTF8&btkr=1&tag=mumsnetforum-21. It's for 3-13 yr olds so your DS is at the lower end but she gives ways to adapt for litte ones. It's really easy to follow and gives lots of examples. I've only read the first section which is about descriptive praise but it has already made a difference. You basically describe what your child is doing right, a lot, through the day.
Even if you really have to search for the good behaviour, just describe any little thing that is OK and name what is good about it for example:
DS, you aren't spitting on your toys - that's playing nicely
DS you have played with DP for 5 minutes without hitting once - very calm and gentle
DS you brought your sister a toy - that's kind and helpful
DS you're walking holding my hand and not screaming - that's very sensible
It feels ridiculous to point out behaviour that should be totally normal and not special but it does work and really reinforces how they should be behaving.