how do i stop my son from taking his own life?

I am going to take this subject to be about the depressive-attacks and catastrophising that the chronically-moody often display, I going assume this for the purposes of making my point, I apologise if that is not at-least an element of the issue that you’re raising. 
Stress and mood disorders, from what I have personally-experienced, are largely-caused by resisting the markers of anxiety in stressful-situations and the over-embracing of fell-thoughts whilst in a low-mood. The most-dire vocalisations that I have ever made, about  morbidity, have been as a result of failure to distract myself from a rumination. The closest I have ever been to dire-action have been in the midst of very-threatening and pressurised situations. 
So my advice would be to ensure that you do not prompt your son to revisit fell-thoughts. Instead listen to your son without interruption when he has something to get-off-his-chest. Reassurance him that depressive-attacks come as quickly as they go. And distract him with his interests and support him when he asks for help.  
I hope this helps, the solution to morbidity is often and most easily-achieved, by shifting ones perspective and having good-timing..

It sounds like he needs professional help, not to be detained but to be listened, heard and supported. There’s kind and understanding professionals who can give him the help he needs and also offer support to you as well. Thoughts of suicide are really serious and they can change from thoughts to actions in a heartbeat, at which point there’s no second chances. So it’s important you get him the help he needs as soon as possible. I’ve experienced this myself and I know what a lonely place it feels like feeling like that but there is help available and though it can be a long haul you do come out better and stronger in the end. Good luck to you both. I hope things get better soon x.

I think we need a few more details about the situation to give truly effective help. A generic "hide the tablets and kinves" only goes so far as su*cide prevention advice unfortunately.

thanks caelus but i think having him sectioned would make things a lot worse, id rather never leave the house myself and keep watch

thanks Iain, i'll take a look at those

I agree with others but it seems like most probably has suicidal ideation but at the moment can’t bring himself to go through with it. You really can’t be expected to deal with something like this on your own and it’s not in your son’s best interest that you try to do so. I strongly encourage you to try and get child mental health services involved. I think if his friends are also from his school it might be worth getting the school involved.

his friends may be aware that he is autistic but it doesn’t necessarily mean that they understand how the autism affects his obsessive tendencies. having authority figures at the School provide clear guidance that any alleged ‘creepy’ behaviour is more likely to be a consequence of autism than any actual malice might help with the situation.

also at this point are you able to rule out the possibility that the situation at school has gone beyond simply falling out with friends and moved on to people actively bullying him? 

if he says he went out and claimed to try and didnt have the energy to do it thats a good thing i think as it means hes not serious about it and its more a cry for help...

the problem is figuring out how you can help him... how you can satisfy him and solve any issues he has.

if your really concerned about it with everything you have said you can have him sectioned for his own safety to prevent any suicide attempt, that way he  can be under monitor at all times in a safe secure location for a period of time, then he may get whatever help they can provide to do with it.

Hello there, welcome to the comunity here.

I'm sorry to hear your son is going through this at the moment and he does seem to be making a cry for help by telling you about it and then backing out from doing it, so that is a positive.

There is some great advice on other discussion threads here which may help - here are a link to a few of the posts with valuable content:

https://community.autism.org.uk/f/mental-health-and-wellbeing/17364/suicidal-thoughts

https://community.autism.org.uk/f/mental-health-and-wellbeing/20368/trigger-warning---suicide

https://community.autism.org.uk/f/mental-health-and-wellbeing/24363/depression-and-suicidal-thoughts

I hope these offer some information that can help.