my 17 year old Son is drinking alone

I have a 17 nearly 18 year old Son, our only child that has Aspergers but only recently diagnosed in the last month.  Is it normal for him to sit in his room on his own drinking alcohol or should we discourage this, he spends most of his time in his room even eating meals.  He also smokes canabis, he says it slows his brain down as he thinks to quickly, we absolutely do not want or agree to him doing this and is not allowed to smoke it in the house, but does anyone know if canabis does actually help with an autistic brain ? sorry i know this is a controversial question but canabis is hitting the media a lot at the moment. He does work as a builders mate so brings in his own money but says he has no proper friends