Is he on anti depressants? @Feelingsmall
The hardest thing is to see is when they refuse help.
One suggestion I can make is to take him to the emergency department when he is highly suicidal. Yes, he may say no, but it's better than him no longer being around. I recognise a 20 year old is not a kid you can pick up and take to ED so the other option is to call 000.
In my area, they send out the PACER team which is a mental health professional who work with the police. They come and assess the person.
Once he is on the local mental health system, there's more scope to get support.
The main thing is that you need to protect yourself and make sure you look after yourself.
I'm hearing how hard it is and how heart breaking it is for your son to ask you to help him end his life. He needs help. Let him know he deserves it. Even if he wants you to go with him to see the GP or something.
tyme