Respite for my son

please can anyone advise, I'm desperate and can't believe I'm publicly saying this. My son is 24 ADHD with Asperger since 5yrs, his mental health is at it's all time low, he has  self harmed since the age of 7 and now I'm worried for that knock on the door to say he's dead. 

Im even starting think he's better of dead and he's begging for me to just let him go, I'm the one who's keeping him here and he doesn't want to keep this existence anymore. I'm starting to consider the next time he threaten to commit killing himself I'm not going to try a plead with him.

Hes had throughout his life false promise of help and support. He's been waiting for counselling for years although we have tried and paid privately but unfortunately they couldn't do much due to his Asperger. He just keep getting let down and abandoned. He's been committed once for two weeks under the mental health to be told it's not serious enough over dosing on every medication you could get, the slashes on his body but apparently he doesn't have a mental health condition. He's worried that if he doesn't get help soon he may hurt others not intentionally but the thoughts that run through his head because he doesn't want to live anymore. He scared of the police I can't turn to them, my local mental health team feels it's just a cry out for support but really can't offer much. He's now taken to smoking cannabis to self medicate for the past 4 yrs wants to stop but can't because of the things that run through his head. He want to go somewhere desperately to try and change such as respite maybe a week or more to help him learn wants going in his head and help him learn to manage. If he was on heroine or harder drugs he can get help. If he drinks acholcol he can get help. I've looked at private place which there are some offering everything he needs at a cost of 5, to 8 thousand. I don't have that money. 

Please help I've never registered his autism but he does have a confirmed diagnosis if there any place that can offer him help. If this continues I'm afraid I would rather see my son in peace. I won't stop him next time.

julie