OP, I'm so sorry you're going through this.
We had a very close relative who had an alcohol addiction and one of their gripes was drink driving. It almost tore the family apart so I completely empathise with your feelings of guilt.
However, this is in no way your fault.
He made a very, very selfish and stupid decision. You could not have stopped him, you could not have controlled him. At twenty you think you're invincible, bad things don't happen to you and there's nothing in the world that could be your fault.
Sadly, he's had to learnt the hard way that that is simply not true. I imagine all of you are very much in a state of shock if not suffering to an extent with PTSD.
If nothing else, the positive of this situation is that this will be the first and last time he will ever drive drunk.
I'm unsure from your post whether he faces a custodial sentence, but the fact he is very reserved shows the incident has had the 'correct impact', as harsh as that sounds.
He feels guilty and he knows what a terrible call of judgement he made - his remorse shows you've done your job as a Mum. Support each other and be kind to yourself, so many families have not been as lucky.
One day this will be a terrible memory that I'm sure will serve as a turning point in his maturity. He made his decision - this in no way is your fault.