I was a sort-of SM (lived together but not married). With the now-ex DP 7 years, lived together 4. DSS lived with us for most of those 4 years and was with his DF a lot of the time before that.
The DM was officially the resident parent (if that's what it is still called?) but she simply wasn't around for DSS, and always put her own relationships first. For a whole year she refused to have him stay with her because her then-DP didn't like DSS, then she split with him and couldn't have DSS to stay with her because she had nowhere to live (was still living in house with her ex) and then she moved in with a new DP in a caravan and couldn't have DSS to stay because there wasn't space....etc etc.
So, we had him most of the time. But I never had any relationship with her because I could not respect a woman who behaved like that (and it turns out I also cannot respect a man who allows/facilities that and behaves the way he did as a result).
When she came to see him I would just go upstairs and read. I did refuse to allow DP to agree to her staying in our house with DSS one time because we were going away for two nights (DP birthday) and she was due to have DSS and said she had nowhere to stay (he paid £500pm maintenance, I suggested she get a hotel for the two nights in 12m she might actually spend with her child). But mainly I just left them to it.
At the beginning DP was all 'let's go round to DSS DM and have a cup of tea and play happy families' but I didn't like that. I never stopped him doing it but wouldn't go. If we stopped there on the way to somewhere I'd wait in the car.
She almost never went to his parents' evenings, sports days, plays, scouts things etc. I would go to the less important things ONLY if she wasn't going (plays and scouts, never parents' evening, that just isn't right).
What she always did want though was him with her Christmas Day. So she could post photos on Facebook of all the presents she bought him. So, come Xmas Eve, as soon as it hit....ooh, about 6pm (when the shops had shut) she would pick him up, then Boxing day she would be so so sad that we didn't collect him until 9am.
I found her very difficult to understand so I didn't try. DSS once said to me "I'm sorry that my mum doesn't care about me" (apologising to me, I can't recall why, it was half-joking).
I never made things difficult between them but I kept out of her way. I was cross one time on his birthday she turned up with a load of presents and I didn't know she was coming and DP let her in the living room with DSS and I was there and couldn't leave without looking rude. DSS was thrilled with his presents - not. All she ever bought him was designer clothes. He liked them more than other clothes (and refused to wear anything we ever bought him) but I don't think he considered it a 'present' (and neither did we!).
I think DP would have preferred if everyone just all got along but he brushed aside her bad behaviour and the effect it had on me (me who provided the home, worked f/t, looked after DSS while he was away for work, did all the organising etc etc) because he has zero empathy.
Funnily enough, now I am away from him (though I am still in the joint home, just taking a while to get move sorted!) I can see that actually they are very similar. He is also neglectful of his DS, just in different ways. They actually suit each other so I would say to the OP - look carefully at your DP and his ex. And don't believe what he tells you (he now tells people the same things about me that he did about her).