Hello there,
Welcome to TeenHelp. It is quite nice of you to be asking for advice on how to deal with your 18 year old son going through a rough patch. It's totally normal to want to seek advice and help on this. It shows you care about your son and want him to do better for himself.
I too think finding a good time to sit down and talk with your son would be the best course of action. I'd discuss how you feel about how he is behaving and acting recently. I also would mention being concerned for him and his health doing these drugs. Being addicted to drugs or any substance is hard and a rough time for everyone involved. We want to see the person be well and seek help.
I also too would speak to the girlfriend's parents about this and find a good time to discuss all of this with them. I also think it'd be useful to mention what you went through as a teenager yourself. Sometimes when a parent went through this, that parent will not usually open up about it or discuss it much. I feel it is a good idea to mention it as he'd have a person who went through the same struggles to talk with.
Apart from this, if he does want help there's resources you and your son could find and go over on our TeenHelp resources page.
http://www.teenhelp.org/resources/#substance_use