We are parents of a 44 year old son who still lives with us. He is very depressed but refuses help.

Is he able to communicate with you about how he feels and why he does not want to seek any help?

Many autistic people, myself included, will have had negative experiences with mental health services and anti-depressant medication in the past. These factors can make us unwilling to be go through the same thing again.

A topic which is currently the subject of much discussion within the autistic community is autistic burnout. This can present like depression and may be misdiagnosed as such by the medical profession. However it is very different and requires a very different approach to recover. A sufferer will seek to withdraw from the world and reduce sensory input as much as possible.

Hi and welcome. I'm sorry about the current situation.. It must be so difficult for you all. 

Is there local support who you might be able to involve? There's a national autistic society near me and they've helped me as I'm in a similar situation. 

Samaritans are good as well, if your son is ok to communicate over the phone. They also have an email service. I understand though not everyone is comfortable sharing how they feel. I really struggle with this.

Wishing you all the best.