Oh Yes, stuck between a rock and a hard place.... as your son is 20 it is impossible for you to force him to engage in any treatment, and at his age, having a job and an independent source of income is important to him.
It is unclear what types of treatments or support you have previously sought, but yes it can take time, and it is not as much about getting a diagnosis (label) as it is about finding the best treatment plan for you son. We simply want our children to feel better.
Losing his apprenticeship has probably made him even more reluctant to be open about his issues, when in fact if his employer had been aware they may have been able to make accommodations, just as they would if he had broken his leg or had some other injury requiring time off. In this case a diagnosis may have been helpful. I am not an industrial relations expert, simply speaking from experience.
Having said that, there are excellent residential facilities available which may be able to provide your son with the information, support and strategies to deal with the symptons of his illness and put him back on a road of some stability. Depending on where you live this may be an option, however it is my belief you will need a referral to these from either your G.P or Psychiatrist (if your son sees one).
You may also have to realise that he has to take contol and responsibility for his own health and wellbeing (a very, very difficult tie to cut), you have also not said if he is living at home and how this may be affecting your own health (which at some point has to become your priority.. again very difficult).
My only advice is hang in there, sometimes you really do have to hit that hard rock bottom before the situation starts to improve (based on my experience), so rather than struggling to hold it all up, prepare yourself for the landing, by allowing yourself to step back, by disengaging to a certain point, from all the excuses/arguments/bad behaviour/whatever it is that your situation is throwing at you, and by asserting your needs to your son, for me is was about being honest with my daughter about her behaviour and the toll it was taking on me. I had tried this reasoning before, many times, the difference this last time, she had to hear it from me in a total state of emotional despair and physical collapse. The difference, I was literally at the end of my rope, my daughter knows I will always love her and when she is ready to be more pro active and to make more positive decisions regarding her illness that I will always, always be there for her, but until such time I can't continue to give her absolutely everything I have and leave nothing for myself.
I sincerely hope you don't have to reach this point, but for me, it has signalled the beginning of a new positivity in my daughter, yes it has only been one good, emotionally stable week, but I'll take it.
Good Luck on your journey, stay strong for your son, be kind to yourself.