Son has spastic quad and autism - head hitting

My son has spastic quad plus autism. He is a lovely placid 15year old. He deals with change etc very well. Just over a year ago he started out of the blue hitting/punching himself in the face and ears. 
After months and numerous visits to the hospital (Alex looked like he had been beaten up) they diagnosed Sinusitis. The behavioural stopped as this was treated.however, a few months later it started again but his sinuses were clear.... to cut a long story short after an abdominal x-ray we found A was impacted and severely constipated. 
We have been dealing with this, however, this time the head hitting mainly at bedtime isn’t subsiding. 
He is waking at 2am and hitting himself.. causing damage to his face and ears. 
We have been giving him a sedation to try to help but makes little difference. 
Everyone is saying the behaviour is due to the pain from the constipation but I’m not sure now as it seems to be going on too long.
I am exhausted and just want the old A back.
any advice would be appreciated. 
I have spoken to several medical professionals and CAHMS who have said his behavioural wouldn’t just change and it must be associated with pain. 
Thanks in advance.