I kicked drugs nearly ten years ago now and he really will just have to keep at it. I still, ten years on, suffer from appallling insomnia and have never slept for more than three or four hours in any given night although you do actually get used to this, but it seems to be my lingering problem. Be there for him, be there for him and be there for him some more. This was the way I did it and if it hadn't been for my wife I'd have killed myself round about days 3-7. After that it got very steadily easier as my noradrenaline levels started to drop ( excercise helps with the overstimulation that causes, if he can stand it).
I would say that he needs to be dealing with any emotional problems etc that lead him to take drugs in the first place or he'll end up back on them pronto as they leave a yawning gap in your life once you are not a user that fills with everything they've been numbing for years so expect some emotional repercussions too. I was a foul person when I was using drugs and suddenly I was hit by this wave of guilt and remorse that was engulfing, so watch for emotional breakdown if he's done some things he's none too proud of or is from an abused background etc.
It is possible to do and once he's past the third week he'll really be so much more in control, but keep him clear of his past associations if you can, as they'll try to lure him back.If he's at the point where he genuinely wants this, then he'll do it. Just be there for him to offer your support.Give him my very best wishes.