From bitter personal experience I can tell you that he:-
a. needs to WANT to give it up, not just say he does.
b. needs to disassociate himself from any friends who are users.
c. Accept that whilst trying to stop he's likely to lapse and forgive himself for that when it happens and just start all over again..... and again.... and again.... etc etc etc
d. will need a lot of support from someone.
Now the only thing there that is really pertinent to you is d. Can you offer him that level of support bearing in mind that to succeed he'll likely have to finish with the new coke snorting girlfriend and there's bound to be hassle there, it will take a lot of time and patience, he'll be a real f3cking mess and not very nice to be with for some time and after all that , with all his and your hard work might genuinely fail in his endeavour.
When I kicked my considerable habits I did them all at once, so it was probably worse than average, but I had the help and support of someone 24/7 and it nearly killed me in all earnestness. If you can't offer him that then best just get in touch with a drugs rehab unit and let him take it himself from there, sad though that is.