I don't have an answer and struggling with this issue myself, expect DS remembers his daddy (as a wonderful superhero, sigh). He is very sad he doesn't see him anymore.
One thing I was told by a child psychologist, is that you need to be very careful about telling a child their father is abusive/ bad etc.
Apparently It's very common for the child to worry that they will also be bad as they are half their dad. That causes fear and self doubt, and they don't have the emotional resilience and fully formed self identity to be able to deal with it.
Simple statements that are limited to the fathers behaviour are much less scary than talking about how bad he is, how nasty he is etc. And never any links from the fathers bad side to the child's behaviour.
E.g. I'm not telling DS that his father stole (a lot) of money from us, as DS stole money out of my purse a while ago. There's no link and DS was was suitably sorry and it's completely in the past now... but DS won't see it like that, and it would be damaging for him to make that link around shared bad deeds. Especially if I told him that's why daddy isn't here anymore, it could make DS feel very insecure and that I'd chuck him out if he ever steals again... basically it's a ruddy minefield!
I had planned to use the stealing example to help DS understand why his father isn't with us anymore, as it's actually one of the least scary things stbxh did and very understandable for a younger child, so was all prepared as a good way to start. Then DS stole and instantly I can't use that example argh!