Sphil, he sounds similar to my ds, very passive but suffers with anxiety and directs it inwards.
It could be a combination of being tired, not drinking enough, anxiety, perhaps even difficulites with sensory processing (god our kids have a lot to deal with).
When Ds was having his 'flash backs' and he also used to think he had 'things' crawling over him and he would scratch and pull at his skin to try and get them off. He used to say that he tought he was an alien and if he pulled his skin off, i would be able to see to. I thought exactly the same as you that he might have mental health problems.
He does in a way, he was suffering with severe anxiety and directing it inwards. He was finding school very difficult. CAMHS explained the alien thing to me as ds feeling very different to everyone else, like an alien, which made more logic sense to me and easier for me to understand.
It was a very worrying time, but it didnt last too long and there is usually a logically explaination, as oppose to a severe mental health problem.
Ask your dr for a referral to CAMHS if it continues.