Things seem to be looking up.
He's still not himself but my son is definitely getting over whatever is wrong.
Yesterday he got a letter regarding funding. He opens all his own post but if there's anything that needs sorting he passes it to me to deal with and he came to me with this one not sure what it was about. That in itself was verging on the positive as he hadn't just put it to one side but come out of his room to see me and I told him it was to do with the letter he'd given me a couple of weeks ago but he couldn't remember it. We were in my room at the time and so I got the first letter out for him and that's when I knew he was getting over his funk.
Now I'm not the tidiest of people but I know where things are so when I had to get the letter out I knew it was between two piles of clean washing that I hadn't put away. Under the t-shirts but on top of the trousers. So I lifted up the t-shirts and pulled the letter out for him. For a second he had a surprised look on his face but then he was obviously amused by my filing system and started to laugh.
His laughter caused waves of relief to sweep over me. A little thing but it showed that he was pulling himself out of it and was on the way. He'll still probably be down for another day or two but last night I got my first proper night's sleep for days.