I also didn't complete my A-Levels due to mental health issues, 15 years ago now. I tried again the following year, dropped out again. There was alot going on in life at the time, and there wasn't as much focus on teens mental health as there is today, and not much support, not that I would have took it anyway. My Dad thought I was doing things just to mess him around and my Mums own mental health had collapsed after my parents separated. Mum was a warehouse operative all her life, didn't see the point of further education when you should just be going out and getting a job, so after I turned 18 and she no longer received maintenance from my Dad for me, as I wasn't in full time education, I had to get a job myself. I never went back to education.
That wasn't the point of the thread, but I felt I had to frame where I'm coming from when I say this.
Even if he never achieves A-Levels, it's not the end of the world, or his life. I don't have a "middle class" or white collar job, there's no prestige in what I do, yet at 31 I have my own house in a nice village, I've had decent cars, I'm in a great relationship and I'm looking forward to meeting my first child in November. This may seem like a brag, but it's to show you, OP, that whatever path your son ends up taking, it doesn't mean he can't have a happy and successful life, with or without a degree.
Focus on his health more than anything. I carried my MH issues well into my late 20's, and it's entirely possible they may resurface again. I went to some very dark places mentally, and I'm aware I'm incredibly lucky to be where I am today. If my mental health had been addressed younger, who knows what path I may have ended up taking.
I also have a friend that didnt go to Uni untill he was 27, after also struggling with things during that allotted 18-24 slot, and he's doing great now after graduating last year.