Hi, I can personally relate to much of this...my main abuser (that I know of, I've had at least 2-3 but suspect abuse I don't remember due to amnesia) was about 9 months older than me, and we were both children when she abused me for years on end. She was physically, psychologically, and possibly sexually abusive towards me. She was vicious, if any of my friends (who she attempted to isolate me from) confronted her about how she was treating me she would threaten to kill them in front of me because she didn't want to stop abusing me.
You're definitely not alone in wondering if your abuse was child abuse, and due to my experience children can definitely abuse other children (I consider anyone under 18 a child). And like you said, he was much more adult like and it sounds like there was a huge power imbalance between the both of you. If you want to see it as child abuse then that's perfectly valid, I myself tell people when asked that I was abused as a child, even though my narrative of child abuse is pretty atypical. I think many of us struggle with feeling like we deserved what happened, I do too, but no matter what, you were a child who should never have had someone there to beat you down and take advantage of you. Children don't deserve that, no child does, not even my own abuser who may have actually attempted to kill.
As for the baby, you probably have limited options. At this point I don't know if you could get his father locked up for the rape, or what situation that would leave the baby in, but it could definitely separate them. Then again, that could cause trauma for the child. It's a tough situation and unfortunately there's probably not much you can do for him.
Keep this boy in your good thoughts if that helps you find solace, I'm sending my best wishes to you as well because this post kind of resonated with me.