Help with boyfriends son moving in

Annelise • Mom of 5

So my boyfriend an I have a place together. I have full custody of my two boys from my previous marriage and he has weekends with his son from a previous relationship. We are also due with a first baby girl in just 2 weeks. We were recently told that his sons mothers husband has been abusing their son. So we went to court to file and ex parte and custody agreement because for the past 8 years of their child’s life they never went to court to establish custody. My boyfriend has just always had weekends and holidays if aloud by her. He was 16 when his son was born and she was 20. So for a long time he believed he really had no chance in court. Plus they started charging him child support at 16 and never aloud him to fight for custody due to his age. His mom tried to get the woman for statutory rape since the child was conceived when my boyfriend was 15 but no luck there. So moving on yesterday the sheriff served the mom with court papers and she freaked out. She called my boyfriend asking what he did and why this was happening. He said he didn’t feel comfortable talking about any of it until they went to court. At this point we are ready for her to flip out. But the complete opposite happened. She said her husband didn’t like being accused of hurting their son and that he never did anything to him. However he wants to move out of the state to his family and she wants to go with him and the child they have together. My boyfriend ready to snap saying your not taking my son didn’t even have to say those words because she said he can have him. She said we can have their son if my boyfriend agrees to let him call her whenever and allow her to see him on vacations. Again my boyfriend said they will talk in court with a mediator. She said okay and continued to ask if the dec 12 court hearing was the only one they had to go to if they agreed on everything. He said they won’t know till they go and she said well either way she’s leaving on the 16th! That’s 4 days after court. She’s just gonna leave. This whole thing is crazy to me. If they weren’t guilty of abuse why are they running?! So now we will have 4 kids full time. I’m so happy for my boyfriend but im so overwhelmed. My boys are 4&3 and his son is 8. It’s gonna be such a change and I’m nervous. Plus I don’t know how to make the transition the best for him. I know he loves being here with us and loves my kids but it’s obviously gonna be different. He’s use to having fun every weekend but now he will be here all the time with all the kids being treated the same. I just need advice on how to make this work because I know his mom leaving is gonna hurt him. He hates her husband but obviously loves his mom. And his mom is choosing her husband and new baby over him :(