You could just deal with it via GP
I would not get into the IEP stuff just say you were very concerned about his mental health while in school & injuries had been increasing since the support was removed - but as you consider that is school related anxiety, you do expect that to improve now he is home
you could point out the injuries are improving already
Be good for the GP to be able to note he has scratches which are beginning to heal over
However even if his mental health does improve he will need CAMHS support to get him back into education, so you think a referral will be needed to support that process / get to the bottom of what is causing the anxiety / self harm and their involvement will be needed if your appeal for statutory assessment is successful
I don't think you need to worry about SS too much, if they were concerned they would have knocked on your door already. They will have to do something to formally close the case but I can't see them wasting precious resources on you. I think they will be annoyed with the school for dragging them into it and giving them paperwork they don't need.
If need be just take him to the GP every few weeks to get it recorded he is improving at home.
That way the GP can write a letter when you get an appointment through for CAMHS documenting that your DS self injury was bad in June but has improved throughout the summer.
I think what Keepon is saying is if you stick to facts and not opinion (including yours) that the evidence will speak for itself
CAMHS can't take a view until they have reopened the case and then its going to be based on their assessment of how your DS presents, not on your opinions as to what school has done wrong or his IEPs. And given the school are saying its all in your head I would wait for CAMHS to ask you questions.
Hopefully as your DS improves then that will be all the evidence you need.
Keep taking the photos / keep a diary e.g. on self injury