This was our situation two years ago.
Please stop being the go between, I did this and I believe that it made it look like Dh didn't care, it needs to be him.
We did mediation, she screamed at dh for both sessions bringing up various things that had nothing to do with DSS. Anything in the sessions is not taken into account in court and is suppose to be for people that get along fairly amicably. It was a waste of time for us and delayed the court process.
Cafcass are absolutely useless, overworked and do not assess situations properly. We had all the info you have, plus a report from social services whereby she was known, reports from school that DSS was suffering as a result of his bio mum being negative about DH I'm front of him and him repeating things.
We have spent over 10,000 on mediation, solicitors and court fees and we have ended up with indirect contact via letter because DSS no longer wants to see us. His mum alienated him and told him things like we hated him and that his grandmother didn't want him to be born etc we went from having him every weekend to nothing.
Bio mum would not allow us to know anything about what he did even if asking casually how he was or if he'd been ill, she didn't tell us anything about an operation he had, took him away without telling us, wouldn't let us have him any other times or take him on holiday. He never spent a Christmas with us or was allowed to see his paternal family without us present and even when we did she'd scream on our doorstep.
She took us to court made false allegations that we'd go to her house and harass her and when she got the above she moved so we had no idea where he was and got a p o box address for dh to write to. She can allow further access at any point as it was proven there was not an issue with violence but because DSS didnt want to come and because emotionally he was very unstable it was decided that indirect contact to then progress to normal contact would suffice.
The weeks after the hearing she kept hand delivering a photo once a month to my house and when found her here she laughed in our faces and told us he hates us and that she got exactly what she wanted which was us out of her lives. She knows we cannot afford to go back to court and does the bare minimum she needs to do and if we were to go back if he said he didn't want to come no changes would be made.
Please please please consider if this is the best route for you. Make sure you have the money and support your family needs. My Dh and I have been through utter hell and there is not one day that goes by that we don't think maybe we should have just put up with the shit to have him back.