partner in denial, emothionally attached

he thinks because they have a child together it gives some type of entitlement to be as confertable as they once were while they were dating. and of course sees nothing wrong in his actons. the lies the intimate confersations the late night visits telling me in hopes to see his son.
I feel great concernment over the fact ive been with my partner for almost 3 years now and he's still defensive and in denial about the woman he shares a child with. It's like I'm battling for the focus to be on the child, which is the main purpose and somehow it always leads back to square 1.
I feel very offbeat about this whole situation. Like I myself have a child, although he doesn't live with me that goes without saying I absolutely understand.
At first I didn't mind, nor had I been aware I was what you would call "rebound" for half out relationship.
I was more than excited he was involved with his child. He invited me over and I guess we ended up staying roughly almost all day, I was ready to leave which I told him and I hinted to him to come (we biked there and I didn't want to bike alone) he didn't and stayed. I was ultimately so confused. A few other times I went I ended up the whole time with his son reading to him when the whole point was for him and his son to visit. While he sat on the floor with the mother of his son talking the whole time. Only then when she started showing up at 4 in the morning or when I realized the first person to ask when he needed something so happened to be her. Don't get me wrong, yes I do think it's a necessity for the two to have some sort of relationship, but I mean more along the lines of "co-parents" not familiarity which they show absolutely and the unwillingness to have that separate. She's not going anywhere, she'll still be there until the child's 18 it's clear the unwilling to recognize his actions is the only reason masking or boundaries for simply a foundation of trust. Something other than what has been showing me. That he can say no or say if it's inappropriate.
He became an absolute issue I found a nude photo of females frontal only 8months later he confessed it was hers.
Times when she didn't even have her son and she started knowing more details on what his day was or what he was going to do. I felt so bad he didn't see his son as much so I offered to invite him to my sister Childrens day party, long story short, when dropping him off with her car he borrowed her by accident kept making slips that he was there the day before and asking for her laundry bag. And it so happens the night I woke up, he said he was out doing laundry and hadn't come home with it turns out he got her to do it.
So when the constant lies became apparent it became hurtful. I had asked there should be no real reason other than your son for you to be talking. Because I felt it was excessive. Every time he asks me out on a date at the time would pass, then he would Fes's up that's where he was.
He had told me one thing and we discussed it and agreed which after insisting I go with him when I do he constantly puts me on the spot when in her presence. EXAMPLE; like basically insisting that I come and then when we're there and she asks why am I here he says didn't bring her and agrees "yeah why is she here"

.
It's very embarrassing and reminds me how much she's right shoes threatened me saying if I ever got in between them id end up in jail like her boyfriend.
His thoughts are that I'm crazy every time her name is mentioned he raises his voice and continues saying there just friends and says they don't hate each other. I am not asking him to hate each other even suggested boundaries because it's clear there are none. The moment they met up they held hands, both while they talked simple as that while I sat in the car was devastating. He's constantly saying, but she's my baby mama, yeah, I get it, but that doesn't give her special privileges. He found it much easier to cut off contact all together than to be able to only talk to her only about his son. Which his son is 8 he is perfectly capable of calling which is proven since he's called my cell. I was more than persistent and encouraging for him to see his son or call which is what gives me reason to believe it's more than that. Impossible he says to tell her some things inappropriate he doesn't want to sound like an ash hole. Yet she insists the only way for him to see his son is if they talk at night when the kids are sleeping at their house. He did not take up that offer until a few days ago to spit. He was there after 12 getting there at 10.
Which bothers me the most is he finds it easier to tell me something AFTER he's done it or constantly says "I'm not hiding anything" just because NOW I can see because he's sharing location that differs from him personally telling me that's he's thinking about it or beforehand so I'm not taken by surprised or hurt. I hate having to deal with this and wish he would make it about his son fir his son's best needs, but it's not he thinks it's ok for her to bring up their past relationship and call him slut in a flirty ways on top of saying awful rude things which he doesn't seem so say stop.