the bottom line is, there can be every reason in the world for your cousin to need to get clean. It may be completely obvious to everyone around him that his world is a wreck. But unless he happens to see his dillema, as we call it, 'a moment of clarity' any and all effort is for nothing. Until you want to get clean, and are willing to do the work necessary to make that happen, you can't get clean.
Do you really want to help him? Let him know that when he's ready, you'll take him to go get help. And when and if that happens, let him help himself. So many times our family members have tried to 'help' us, and in doing so, only made it easier for us to continue doing the very things we were doing. Your family has the right idea(whether they know it or not)- they're not doing anything for him that will make it easier for him to keep getting high. That usually means no sleeping on the couch(just for the night, you know), no buying him any of his personal needs(because thats just one less thing to worry about for him, so he can spend more of the money he has on dope), and for God's sake, no cash to get him by for the day or whatever it may be. Any assistance you give makes it easier for him to keep doing what he's doing. You really want to help him? Don't help him. The faster he hits bottom on his own, the faster he may get on the road to recover. And that's no guarantee, either. Addiction is powerful. You yourself said that he'd already been to at least one other treatment. So its not as if he doesn't know that there is a solution.
So, sorry, Abozer, there is no promising input to help get him on the right road. Its just the nature of addiction. Some have a saying, hope it makes sense to you: When the fear of change is no longer greater than the fear of remaining the same, change will come.
There is one thing you can do for him- you can pray for him.