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I'm sorry you're having such a difficult time with your son. Aside from the options you've mentioned trying, you could look in the Autism Services Directory here on the website - it depends where you are in the country, but you may be able to find help from a non-NHS source that would be able to help more urgently. You can find that here - http://www.autism.org.uk/directory.aspx
Alternatively you can speak to someone directly on the NAS Helpline at 0808 800 4104 (Mon-Thurs 10am-4pm, Fri 9am-3pm) who might be able to suggest other options.
Ross - mod
Also the NHS page on suicide has several useful resources for people feeling suicidal and their family members - http://www.nhs.uk/Conditions/Suicide/Pages/Getting-help.aspx
And Samaritans operates 24 hours a day, and you can email if you'd rather not talk on the phone - https://www.samaritans.org/