When i was living with my ex and trying to decide if he was abusive or not I used to go on and on about context. Its all very black and white when you write posts out; my situation with my parents reads as very extreme compared to your situation but at the time it never feels that extreme.
With my mum and SDad, things shifted slowly over time so that I didnt really notice that he stopped paying me any attention as I was busy with after school clubs, homework, exams, being out with friends etc. It slowly dawned on me that he had started to be "off" but put it down to all sorts as did my mum. For years I accepted the argument that he found it difficult to relate to a teenage girl, felt awkward around me when I was hormonal and having teen strops, didnt like to see me and mum arguing and so "withdrew". As an adult I can see that he just didnt give a toss, resented my presence as soon as mum became pregnant, slowly phased me out of his new little family.
When I now post about the situation its with the hindsight of knowledge - knowing that I can condense it all into a few paragraphs and knowing that the problems I had were due to that situation.
I didnt wake up one morning and say to my mum "I am underweight, have stunted puberty, am being sick all the time, hate my SDad and want you to do something" because life doesnt happen like that. I had lots of little conversations just like your DS had with you where I said that I wasnt happy. I was 14, I didnt understand that being sick all the time and weird periods was related to the stress I was under so i wasnt able to connect them. My mother was and chose not to. She watched me progressively getting worse and becoming more and more unhappy and isolated over a period of years and refused to join up the dots. Even when it had got to a point where it was staring her in the face she still couldnt add it all up.
She should have listened before the damage was done. If a child is unhappy at school we listen and we consider going to talk to their teacher/ head of year etc. We work out how we can fix this. So why should home be any different? You dont need to wait until your child is damaged to the point that they are ill, you dont need to read extreme ppsts on MN and think "well its not that bad and I wont let it get that bad" before you act. Damage control is far worse than prevention. The effect of my childhood on me is long term. I have no boundaries or understanding of red flags (without reading up and reteaching myself). I sought male approval throughout my late teens. I was sexually assaulted, I was raped and I waltzed into a highly abusive relationship. One of the first things I said to my ex when he tested the waters with EA was "No matter what you throw at me or however much you lash out I will not walk away from you, I will not give up on you" Which translated to him as "hello I am a human punchbag, please abuse me".
The damage that is done to DC just by allowing them to witness dysfunctional and EA relationships shapes their own boundaries, what they will and will not accept/how they will treat others. Girls are more likely to be abused, boys are more likely to have abusive traits. I watched my ex go from a damaged teenage boy to a cold, manipulative, controlling man. He had been witness to abuse and had been beaten and sexually abused as a young adult.
The information is all out there for you to take in at whatever pace you want to. Its hard to accept, believe me I know. But its harder still for my mother to understand how Ive turned out the way I have because she cannot accept what she allowed to happen. She will lie to herself until she dies because she cannot cope with the fact that she failed me as a mother time and time again.