We as parents cannot make out children do anything and if he doesn't want to get clean no amount of bugging him is going to help. The hard truth is he has to want it himself. Giving him suggestions may get him thinking and let him know he has options but again, he has to want it.
Maybe telling him he has some options if he chooses to get clean and then letting him know you are no longer going to help him in any way, shape or form till he is serious about getting clean would be a good idea for you. Once their life lines get pulled, reality hits them and makes it a bit harder for them to do the things they easily do with a parent's "help."
From one mom to another-pull the plug and make it uncomfortable. That is what will really help.