7 year old son - please help.

A short disclaimer: I experienced mental health problems much older than your son is now, and to a less severe extent. I am also able to communicate verbally and both cope and mask quite ‘well’. What I discuss in my comment is based off my own experience with self harm (s/h), and may not be best in this situation. If anyone has input that is more relevant to OP’s situation, that would be welcomed. This is obviously a very triggering discussion and I have found the ‘CALM’ helpline useful in the past (they have a text service).

For me, I now realise a lot of the s/h behaviour actually came from unmet sensory needs. While there was a significant emotional component, the feeling of pain gave me body awareness that my brain was sorely lacking. My s/h behaviour when I was older was very structured and rigid, but even as a young child who did not s/h I distinctly remember hitting my head against the wall because the different textures/movements compacting (skin vs skull) was a novel and different sensation. Both reduced vestibular awareness and reduced sensation are often present in autism. Reduced sensation is not necessarily the same as being touch averse, for example in my case I struggle with noticing food on my face, but if someone was to touch my face I would experience it very intensely (this is due to a mixture of factors, like the transient nature of touch + anxiety). 
There are a few suggestions online and in this community I’m sure about things that can help this awareness- like sensory swings, playing on equipment/trampolines, water play (depending on his opinion on water), there’s a lot out there. You could also try harm reduction techniques, like snapping a rubber band or using a stiff brush, although the fixation on ‘experiencing a specific harm’ meant I personally struggled to implement this.

I’m also a huge believer in horses for therapy, Riding for the Disabled may be suitable (depending on the severity/frequency of the behaviour, as it is often facilitated by volunteers who are not equipped to handle challenging or unsafe behaviour) or hippotherapy - the textures of the horses and their breath/sounds is great sensory feedback, and their movement stimulates all the balance/coordination systems. Unfortunately both do tend to have a long waiting list, and if your son is very sensory averse/anxious it might not be suitable, I just want to mention it as it’s truly incredible in my opinion.

I hope you can all find a solution that works for everyone soon.

Thank you so much CM. I'm sorry you have had struggles as well. 

It's very interesting about sensory needs. 

He has seen an OT and does have quite a few. 

I will have a research into this. 

Thank you

Thank you so much CM. I'm sorry you have had struggles as well. 

It's very interesting about sensory needs. 

He has seen an OT and does have quite a few. 

I will have a research into this. 

Thank you