3 year old not adjusting to fathers new partner her baby and their living arrangements.

Not sure what else to do other than seek advice here, I have a 3 year old son with an ex partner. He has never really been 100% Involved, sees my son when he says he has time (never makes the time for my son) and when he does he sits on his phone the whole time texting his girlfriend or whatever else he does. My son lives at home with me and my partner and adores my partner they do everything together. His father has recently got a new girlfriend and have already moved in together, my son has met her once before they moved in and she also has a baby. Getting to the point, my son has expressed feelings of anger and upset towards daddy and the new baby at daddy’s house at nursery and to me and my partner. During a parents evening we had the discussion that no one wants to have admitting that their child’s father isn’t doing the best he could with his son. This breaks mine and my partners hearts every time we talk about it. Anyway, my sons nursery teacher advised that maybe as his father doesn’t see him often when he does it is just father son time not my son being forced to be around his new girlfriend and her son when all he’s craving is his fathers time. I expressed these feelings to him and all I got was abuse and how I’m just having a go at him. My son is hurting I can tell from his behaviour and so can his nursery as he expressed his feelings to them also. I am getting to a point we’re i am sick of his lack of efforts and poor excuses as to why he can’t see my son. I’m now stuck with what to do as I do not want to put my son in situations that are causing him stress and upset but at the same time I don’t know we’re my rights allow me to decide what happens. I am 20 weeks pregnant my son couldn’t be more ecstatic to have a baby sister (always kissing my belly putting his ear to my belly saying he can hear her talking to him, placing his toys in her bed saying she can have this and that, it’s all he talks about at nursery to everyone) and at the same him he’s feeling so upset about this new

baby at daddy’s. I don’t want him to feel like this anymore. Any advice would be amazing, sorry for the long post! 😫 also not sure what group to put this in!!