I'm sorry for the pain you are in, treading.
my opinion, is that suboxone is a kind of "last resort" intervention. when they have tried and repeatedly failed, it may be time to discuss using it. because, it is also addictive, and because some people have such a need to get those endorphins flowing that they continue to utilize what is at their disposal (e.g. opiates) so their brain may truly need the artificial help to do so.
i thought that finally, after 30 years of a heroin addiction, on the wagon, off the wagon, on, off, that my xabf would finally have the tool that had been missing, to help him succeed. he went on methadone maintenance. but he only went to a handful of meetings, a couple of health realization groups, and started therapy, but he didn't go "to any length". he is now pretty much just maintaining with the methadone, but mostly a shell of his former self. the mind, and will, is the most potent component in acheiving and maintaining sobriety. it sounds as if his mind isn't even close to being on board.
my hunch is that he believes that the sub will be the magic bullet that will keep him away from the street drugs, or possibly he doesn't even want to stay away from the street, and this will "help him along" so to speak. people do still abuse even while on suboxone.
i think you need to detach from his addiction, and if it's at all possible, do not allow him to live in your home. you guys say he has to have a plan, but the plan, at the start, is simply words spoken, or written on a piece of paper. it's the follow-through that matters, and once he's there and not following through....what alternatives will you two be left with? you'll be the drug/plan police, and your lives could very well be turned upside down even more than they are right now.
i think he has to do this on his own, with you two rooting from a distance.