I think this is one of the most difficult situations for a parent to manage. At 20, your son is a legal adult. You can’t make him do anything. But, at 20, he isn’t really mature. He isn’t making good decisions, and he isn’t open to hearing loving guidance from a parent who loves him.
He may need to lose the job to get the wake-up call he needs. It would certainly be helpful if the HR department would give him constructive feedback and a probationary period (something you can’t do).
You didn’t mention whether he is still living with you. If so, you may have a bit of leverage. You could, for example, insist that he see a psychiatrist for an evaluation as a condition for staying with you. You could remind your son that an evaluation is just that — an evaluation with some recommendations. He isn’t committing to doing anything differently if he doesn’t agree with the recommendations. But just maybe he’ll get some useful information. You can also make it clear that he must have a job and contribute financially to the household.
It’s hard to watch a kid we love lose a job, alienate friends or generally not take care of himself. But sometimes we do need to take a step back and let natural consequences take place. In the meantime, what you absolutely can do is love him, remind him that you do love him and assure him that you will always do so, even when you think he is making a mistake.
I wish you both well,