son 24 recently diagnosed with ASD and has drug addiction

Any advice would be appreciated. My son aged 24, has recently been diagnosed with ASD, which explains why he has been self medicating with cannabis since the age of 14 and the last 5 years with cocaine. He has sought help with our local drug service and now doesn't use cannabis only cocaine and is attending weekly meetings, but he feels that this won't help in getting clean, as he takes it to cope with his ASD. He has been assessed by mental health and the ASD service, who cannot assist him with coping mechanisms as he is on drugs. So my son cannot get any help with his ASD because he's on drugs. We are both despairing and any advice would be appreciated.