honestly?
let the legal side run and concentrate on getting help for your son. if he still hasn't been diagnosed ten years later then there was obviously an element of doubt with a definitive dx.
the notes from school were surely kept under a child protection remit? if they weren't passed on, then presumably it was because school did not have enough cause for concern - ie they were content that ds was safe in your care? from what the sw said, they had been asked to do this by ss? and the investigation was terminated when they were provided with proof of a medical condition?
i don't know enough about cp in schools to know how the reports should have been kept, how long for, and when destroyed tbh, but there are a few posters on here with that expertise. mrz might be one of them.
i know it isn't nice to have found out you were investigated by ss, but you should be slightly reassured that they found you entirely without blame and that your son was safe in your care. so, i kind of think that they were doing their job. as were the school. there are loads of threads about cp on here, try an advanced search to see if you can find any info on how long reports should be kept for etc. i'd be willing to bet that if it was ten years ago and the investigation was terminated that some might have been destroyed.
the needless suffering for your son is being dealt with by the medical negligence claim. you want an apology because you were wrongly accused (except you weren't, as no-one accused you). what do you want the spology to say?
i can tell you what it will say now, and it will probably be of no comfort at all - it will be along the lines of - 'due to mis-dx of your son, it was our responsibility to carry out an investigation into his family life in order to determine if he was safe. in these circumstances where knowledge of the investigation might prove unsafe for the child, we are not required to disclose to families. our investigation proved that ds was not in any danger and had a caring family who were seeking support for his medical condition. we understand that you are unhappy that our policies meant that we were obliged to investigate your family, but hope you understand that our primary consideration is for the safety of the children it is our job to protect. we sincerely apologise for any distress this has caused.' or something along those lines. would it be worth the heartbreak of going over it all again? (genuine question. this stuff can be really bad for your mental health)
you won't get anything from health (the dr) until the med neg claim is done. and what you get will depend on the outcome. the pct will handle it all. absolutely pintless to expect an apology from (him?). how long are you into it now?