It is good to meet you, and thank you for posting your question. I can see you are feeling quite stressed about your son's situation.
I wanted to post because I am 26 and not only do I have some pretty recent memories of my own mother trying to help me with my future, and my partner left school when she was 16. So I guess I have a little bit of experience on the other side.
First of all though, good on you for trying to help out your son and reaching out to us. It can be really hard to deal with, and I really hope you can find what you need on the forums here.
Perhaps the main thing I can suggest is that your son may not want to go back to school at all, and it may not be helpful for him anyhow. It sounds like he did not have a good experience there and my partner did not either. She is dyslexic and after years of dealing with the frustrations of this at school, I do not think she has much desire to go back into academic schooling. If she were to go back, it would just bring back up feelings of inadequacy and frustration. She may go back later one day, but neither of us can see it happening any time soon.
There are many ways for your son to build his own future without needing to go back to school straight away, or even in the near future. It is a shame that the 2 apprenticeships haven't worked out, but perhaps future ones will click.
In the meantime, perhaps your son hasn't formulated a plan of what to do next because he is trying to work on his mental state. I had depression too and it can really put the brakes on our ability to focus and put plans together, let alone carry them out.
Just on your questions, I have heard of people go into tertiary level education with some bridging courses, after leaving high school early, though I do not know about going back into tertiary. It may put your mind at ease to have a little bit of research for your own benefit, but if your son has just had a relapse in the last 3 weeks, I would suggest not raising this with him unless he asks first. It can be very overwhelming.
Can I suggest you have a read of this page? There is some useful information there.
I hope to be able to keep speaking with you.